Collecting US stamps can be dangerous to your financial health (unless you are careful)!

There are a large number of modified and fake US stamps on the market.  eBay contains its share of these stamps...but probably not any more than what you might find at the typical stamp bourse.  You can protect your investment in US stamps by following some simple rules.

What are common problems?

1. Cleaned manuscript cancels to make stamps (mostly the classics) appear unused.  This is a particular problem with 'unused, no gum' classic stamps.  A variation of this is a stamp with a cleaned manuscript cancellation AND a fake handstamp cancellation.
2. Repaired stamps.  The repairs can be of many kinds:  sealed tears, filled thins, added perforation tips, pressed creases, and doctored designs where either additional features are 'painted' in or small design characteristics are scraped off.  There is nothing wrong with a repaired stamp as an addition to your collection as long as you are aware of the repair (except that its market value is significantly less).
3. Regummed stamps.  Note that a hinge mark DOES NOT signify that the gum is original gum. Regumming is quite common on more valuable and earlier issues.
4. Reperforated stamps.  Many flat plate printed stamps were produced with straight edges on one or more sides of the plate.  Up to 19% of the originally produced stamps had straight edges!  These straight edge stamps generally have a much lower market value.  In addition, a significant proportion of stamps originally had less than ideal perforations.  Reperforated stamps are common when the stamp has a significant catalogue value.  There are some incredibly well executed reperforations on the market (some of them even have 'good' certificates).
5. Fake cancellations.  Scarcer cancellations (such as the St Johnsbury Vt scarab and the many Waterbury Conn cancellations) are often faked.
6. Misdescribed stamps: some US stamps are very difficult to identify, particularly the large banknotes where the differences are subtle color shades and paper types.  Watermarks are particularly difficult to see and identify on certain colors such as orange.
7. Color variants.  Scarcer color variants such as Scott 64a (pigeon blood pink) & 64b (rose pink), and the various red violet shades of Scott 537 are very difficult to distinguish unless you already have access to copies to use for comparison.
8. Grills: The grilled US stamps present particular problems because, in addition to the existence of faked grills, the production quality was poor.  Partial grills of one type may appear to be another type.
9. Removed perforations.  Some perforated stamps, particularly if they originally had large margins, have been trimmed to resemble a scarcer imperforate variety.
10. Doctored covers.  Scarcer classic covers, particularly those with special interest to collectors such as internationally used covers, may contain perfectly genuine stamps that are still on the cover as originally mailed.  However, their value can be enhanced with the addition of fake markings and additional cancellations.
11. US Local Issues.  Many, many examples of these stamps are reprints or fakes, even if the catalogue value is only a few dollars.  Larry Lyon's books are more than worth the cost in helping you to avoid these reprints and fakes.  (I see newly listed examples of these almost every day on eBay!)  For the less valuable US locals, a certificate doesn't make economic sense...but you should definitely acquire copies of Larry Lyon's books as an aid to separating the fakes from the real thing.

The rules:

1. If a stamp or cover is worth more than $500 (catalogue value adjusted for quality), you should not purchase the stamp without a certificate or an extension for a certificate.  Why $500?  The certificate will cost a minimum of $40 to $50 dollars including shipping.  In my experience, about one in ten stamps worth $500 have problems not easily recognized.  Thus, $500 is the point at which the risk of keeping an uncertified stamp exceeds the cost of the certificate.
2. Certain issues, such as those Washington-Franklin flat plate coils having imperforate versions of the same design and watermark, are faked so often that I suspect that the available fakes outnumber the available genuine copies!  You should get a certificate unless the cost of the certificate exceeds the adjusted catalogue value of the stamp or stamps.  I have received adverse certificates on WF flat plate coils cataloguing $10 and less!
3. Unless you have access to reference materials for color comparisons, scarcer color varieties worth $100 (catalogue value adjusted for quality) should have certificates or be purchased with an extension for a certificate.
4. Covers having copies of US local stamps should be certified or be purchased with an extension for a certificate unless they are worth less than $100.

Certificates and Extensions

There are three primary authorities for certifying US stamps:

• The Philatelic Foundation - issues the 'most acceptable' certificates as measured by the major stamp auction house realizations.  But...they are generally slow in returning an opinion (90 days or more is not unusual).
• American Philatelic Society - the quality of their certificates has improved in recent years, particularly since they have acquired some sophisticated equipment for examining the stamps.
• Professional Stamp Experts - a commercial enterprise that issues certificates and which has been the innovator with graded certificates.

None of these organizations are perfect, they all make mistakes from time to time.  In general, however, the quality of the work is quite good.  Use Google to find the web sites and additional information regarding these expertization services.

In addition, W.R. Weiss of Weiss Auctions, Bethlehem PA offers much less expensive certificates.  These certificates are not as authoritative as those from the three organizations listed above primarily because they represent the opinion of one examiner (the other organizations typically use three different examiners for submitted stamps and covers).  However, if you are a novice collector, you will probably benefit from submitting some of your less expensive (under $500 in value) to W.R. Weiss, particularly as an educational exercise to help you to learn how to identify modified stamps.

Buying stamps that already have a certificate

Some stamps are sold with a certificate.  However, the older the certificate, the less confidence you can have in the opinion if for no other reason than the stamp may have suffered damage since the certificate was issued.  There are a few fake certificates around as well so carefully compare the certificate to other certificates in your possession.  The major auction houses generally consider certificates issued in the past 5 years as adequate in that they do not allow extensions on such stamps; they allow extensions on stamps having older certificates.

Buying stamps that do not have a certificate

When buying more valuable stamps not having a certificate, you should ask the seller for an extension BEFORE committing to the purchase.  You should also determine who pays the certificate cost and the associated mailing costs (to and from the certificate authority) BEFORE committing to the purchase.  If a seller offers an extension, this means that they will definitely refund the purchase cost in the event of an adverse opinion. Some sellers will refund the certificate fee in the event of an adverse opinion and some will refund both the certificate fee and all shipping costs.  Many sellers WILL NOT refund the purchase price in the event of a 'no opinion' certificate (this means that the experts couldn't make up their minds regarding the genuineness of the stamp or cover).  An adverse opinion is an opinion that differs from the original description of the stamp as submitted.  An opinion mentioning damage on a stamp submitted without a description of the damage is an adverse opinion, even if the identification of the stamp is perfectly accurate.  If you bought a stamp where the seller mentioned damage of some kind, you must submit the stamp to the expertizing authority with a description mentioning this damage.

In my opinion, sellers who will not stand behind their offerings by granting extensions shouldn't be trusted.  Sellers who are members of the American Philatelic Society are required by the APS code of ethics to offer extensions unless they clearly state that the stamp is sold 'as is'.  Furthermore, it is my opinion that honest sellers will refund all shipping costs as well as the certificate fee in the event of an adverse opinion.  If a seller won't refund these shipping costs, you should subtract them from what you are willing to pay for the item.