Why eBay is a good place to buy higher value US Stamps?
From a seller's standpoint, eBay is not the place to obtain the highest sale price for valuable material.  Traditional auctions will usually result in a higher selling price...but the seller must pay a significant commission and wait a considerable amount of time to receive payment.  Sellers who want to convert their stamps into cash quickly will often list them on eBay.  This means that you, the buyer, can pick up some great bargains on eBay.  And, because some sellers are not particularly knowledgeable regarding their offerings, you may find some surprisingly valuable stamps at truly bargain prices.

But...
You must be careful when buying higher value stamps on eBay just as you must be careful elsewhere.  Some eBay sellers are not philatelic experts and they will misidentify or mis-describe their offerings.  In my experience, items are no more frequently misidentified or mis-described on eBay than they are by dealers at the typical stamp bourse.  Unfortunately, this isn't saying much as there is a lot of misidentified and mis-described material on eBay and at stamp bourses.

You can safely purchase higher value US Stamps on eBay if you follow some basic rules:

1. Don't buy items based strictly on the description and don't buy items where you don't have knowledge of the particular era or issue of US stamps.  Spend the time to research the stamp in the literature before buying.
2. Don't necessarily trust the seller's description of the item particularly with respect to identification, quality, or catalogue value.  Use your own judgment regarding quality.  The value of a stamp is as much a function of the quality (centering, gum condition, paper condition, color, size of margins, condition of perforations, cancellation, etc.) as it is of the catalogue number.  Educate yourself regarding the US stamps market and use your own judgement regarding quality.
3. Be extremely careful when buying stamps from a seller who doesn't offer a money back guarantee.  Scanned or photographed images cannot accurately represent all of the qualities of a stamp.  You may discover that there are problems with the stamp after you receive it from the seller.  If the seller offers a money-back guarantee and has a good feedback rating, you don't have a problem.
4. Consider using an expertising service for valuable stamps.  If the stamp already has a certificate, be aware that condition may have changed if the certificate is more than several years old or if the stamp is in a state other than as described in the certificate (example: an off-cover stamp that was certified on-cover).  If the stamp does not have a certificate, be sure that you understand the seller's policy regarding certification BEFORE bidding on or buying the stamp.  Some seller's describe their policies regarding certification in the item description.  However, some do not and some do not list their policies in sufficient detail.  If the policy is not clear, be sure to email the seller in order to determine the exact policy regarding certification:

• Who pays the certification fee in the event of an adverse opinion?
• What is the result of a 'refused' opinion?
• How long after the sale may an item be returned in the event of an adverse opinion?  Some certification services (particularly the Philatelic Foundation) can take a long time (90 to 120 days) to return an opinion.
• Are shipping and insurance costs (to/from the seller and to/from the expertising service) included in the money back guarantee?
• Sellers who adhere to the APS code of ethics will offer a money back guarantee for items with an adverse opinion.  While the ethics code does not require the seller to pay the certification fee or shipping costs in such a circumstance, most reputable sellers who are members of the APS will do so.  Members of the APS are required by the APS to follow the APS code of ethics in order to maintain their membership.
  

Dangerous Territory:
There are certain areas of US Philately that are particularly treacherous.  In many cases, the current seller did not produce the fake item and is not aware that it is fake.  The dangerous areas:

• Washington/Franklin flat plate coils.  The majority of examples of ALMOST ALL of these issues are FAKES typically produced from the imperforate sheets of the same issue!  Many of these fakes have been around for years and have changed hands several times.  Don't buy these without a certificate or an agreement that the certification and shipping costs will be reimbursed in the event of an adverse opinion.
• US Local Issues:  There are many, many fakes of these issues in circulation...even of the relatively inexpensive examples.  Use a good reference such as Larry Lyon's books in order to determine if an offering is genuine and obtain certificates for the more expensive items.
• OG and NH stamps:  Regumming and 'corrected' gum is not uncommon on more valuable stamps.  Look for perf soaks and unnaturally smooth or rounded perf tips when looking at images; these are signs of regumming or 'corrected' gum.  Be sure that the stamp has a money-back guarantee.  When you receive the stamp, compare the gum with that of other stamps of the same issue that you already have in your collection.  The gum color and surface should be similar to other stamps of the same issue if the gum on both stamps is genuine.  And get an extension for a certificate for more valuable stamps.
• Cleaned classic stamps:  many early issues were cancelled using a pen and ink.  It is not hard to remove the cancellation with an ink eradicator although the cancellation will typically show under ultra-violet light.  When you receive the stamp, check it under UV light for cleaned cancellations.  Be sure to get a money-back guarantee or an extension for a certificate on more valuable unused classic US stamps.
• Reperforation:  the original production of flat plate (all US stamps prior to 1918) produced sheets that contained 10% to 19% straight edge stamps!  You don't see these proportions in the existing copies today because many of these straight edged stamps have been reperforated.  There is ample literature regarding the detection of reperforation and most sellers use a black background for their stamps so that you can clearly view the perforations in order to judge their authenticity.  However, some of the reperforations are of a superb quality and you should get a certificate or an extension for a certificate with the more valuable stamps.

In Summary:

You can safely purchase higher value US Stamps on eBay if you follow the suggestions above.  And you may find some real bargains compared to what you might have to spend for the same stamp at a traditional stamp auction.