First let me tell you a little about myself.  I have been collecting microscopes and microscope slides (and some other antique science intstruments) since before Ebay existed.  The hobby has changed markedly since then, in some ways for the better and in some ways for the worse.  Some things have gone up in value (antique slides), while others have dropped in value (most antique scientific instruments).   I seldom sell microscopes but at one time I had sell most of my collection just to pay the rent, so I have been at both ends of this hobby.  Fortunately, over the years I recovered some of my losses and have slowly been able to obtain a few things from friends and also from Ebay. 

Antique microscope slides have really made a huge turnaround over the years. There was a time when they were all relatively cheap and easy to come by and you could buy them litterally by the box (if we had all only known just how cheap they were!).  Unfortunately, they have now rapidly appreciated in value as they are one of the "in" things to collect.  Some buyers are literally willing to spend anything to get certain slides.  Among the most valuable are slides by famous preparers, usually from the nineteenth or sometimes early twentieth century.  Categories include animal, vegetable and mineral and of course many different subcategories within each.  I will try to give you an honest and accurate appraisal but be aware the market is changing almost daily!  As an example, when I first got into this hobby, chemical slides for polarized light viewing were at the lower end of the scale.  Although some are very beautiful under the microscope, they were common and were never hard to prepare.   Now, there seems to be no end to how much some will pay for these.  On the other hand somethings which were always the more expensive (albeit even the most expensive was cheap by todays standards), are still expensive, just more so.

Among the most expensive and valued slides are microrulings, microwritings, and microscopic arrangements tantamount to art.  Mind you among the microrulings, I am not talking about simple micrometers but complicated micrometers, generally with extended ranges often into the many thousands of lines to the inch, or ornate patterns of microrulings; simple limited micrometers are cheap.  Similarly books or phrases, written as pages or even whole texts scribed with a micro-engraving machine have skyrocketed in value.  I have seen relatively simple examples of each of this type of slide sell for many thousands of dollars on Ebay. 

Microphotographs are another hot item these days.  The inventor was J.B. Dancer and his slides are among the most popular and usually command a high price-currently averaging about $100 each and more if rare or in particularly great condition.  Some microphotographs not by Dancer may command even higher prices, especially when of high quality and especially if rare.  These may easily reach into the greater-than-1000 dollar mark.  Dancer also sold chemical slides for polarized light; like other polarized light slides they seem to have gone up in price recently.  On the subject of microphotos I should mention that Anthony DiDonato is a modern contempory microphotograph preparer.  Although his slides are "modern" they are often not modern subjects and usually are higher quality than any Dancer slide, and well-worth acquiring if you do not insist on a "antique" for their entertainment value.  His slides are a good way to get a good microphotograph without selling your house!  He often sells his slides under his Ebay handle as tamagno.

Another increasingly valuable slide are slides of whole insects, especially when a good specimen (either macroscopically or microscopically or both).  Again, some favorites stand out.  Good examples of Fairy Flies always command a good price and if by Fred Enoch, the greatest insect preparer in history, even more.  Recently, (as of April 2008), the minimum price for a whole insect by Enoch has been about $100, and for rare or exceptional examples, prices up to $475 have recently been reached.  To this day no one has mastered the ability to mount an insect with the impecable attention to preserving every detail as Enoch did.   What some people do not know is that Enoch made both slides prepared without pressure, sometimes in a liquid medium, and others that were not prepared without pressure.  The ones prepared without pressure are in general, more valuable and more interesting.  Whole insects usually command a higher price than parts, with few exceptions.  Although the large whole insects are often more impressive grossly, the very small whole insects may be the most interesting under the traditional microscope and knowledgable collectors may pay alot more for these, especially when interested in the under-the scope appearance.  A low-power stereomicroscope is best for the larger specimens.  Good Enoch slides are always expensive on Ebay or anywhere else; indeed this is a case where they may be much cheaper than from a dealer.  This is not always the case on Ebay, but for antiques of science, often is.  (On the other hand, more modern items sometimes cost more than at a retail store!  I recently saw a wrench sold at a home improvement chain for $24 selling for more than that on Ebay just because people got excited and bid it up higher than retail, not once, but for multiple different auctions by different sellers).  

ANTIQUE MICROSCOPES

I have been collecting antique microscopes now for about 18 years.  I started long before Ebay existed.  At that time you could get an antique microscope from a professional dealer (almost always the most expensive), from auction at Sotheby or Christies in England (sometimes cheap, sometimes expensive), or most reasonably, from a friend or aquaintance who is also a collector.   Times have changed and with them, the price of microscopes.  Some have gone up and others down alot.  Scopes which knowledgable collectors thought rare proved to be common when Ebay started and the price dropped dramatically.  A good example is the "Model" microscope by Bausch and Lomb (B&L).  I paid $1200 for one from a dealer before Ebay started, and now you can get them for $200-400 on Ebay.    The law of supply and demand applies particularly on Ebay; the supply is often different than we thought! 

When collecting microscopes several features come in to play and should affect your decision on how much to spend.  These include age (in general, older is more expensive), rarity, condition, original quality of construction, and demand which may outweigh some other characteristics.  This leads to an interesting mix and some microscopes have gone down a lot in value while there seems no limit to what some will spend for others.   Also as time goes on and more people collect, the scopes which are attractive yet scarce, steadily go up in price.   A word of caution however; Ebay is not always the cheapest place to find an antique microscope.  Recently I saw the sale of a Powell & Lealand number one on Ebay.  The seller had just purchased it for about $5300 at auction in England, and it sold for over $10,000 very soon afterwards on Ebay.  It was a good but not superb example as it had no accessories at all yet it sold for a great deal of money.  This occurred because a) P&L stands are generally considered the best quality instruments of the 19th century, b) relatively few were made c) they are attractive, d) they are of historic importance and e) every serious collector wants one!.  The buyer probably did not know of the recent auction and these scopes rarely show up for sale especially on Ebay.   The price paid on Ebay would have been similar to a dealer price, if you could find a dealer with one to sell!  The bottom line from this part of my discussion is you really need to know your subject before you buy an expensive antique microscope.   This will take considerable effort unless you know somewhat that knows.  I heartily recommend you join the closest Microscopical Society to gain some insight quickly.  If not, read on and I will try to give you some more pointers. 

This is a work in progress so pardon me as I stop to update at a later date.