Before I purchase anything, be it online or in a brick and mortar store I research the quality and price. In the case of machines and tooling I check with local industrial suppliers, online retailers, and local surplus stores. In most cases the surplus stores are asking in the neighborhood of 50% of new.
The caveats being:
You already know the item is used, and therefore will probably not last as long as a new item under the same working conditions.
There are likely to be cosmetic problems
There will be no support in the form of replacement parts
In almost all cases items are purchased in "as is" condition
It's up to the buyer to determine whether a used tool or tooling will meet their requirements. If it does then they have to also have to decide whether the difference in the asking price is worth taking the chance. Are you willing to pay full retail for a known tool, of known quality, from a known vendor, or would you rather take a chance on a used tool of unknown quality from an unknown vendor for a fraction of the price of a new one?
I have no doubt that eBay is hard to deal with, however what passes as a "fair" price in one area of the country doesn't necessarily pass as the same thing in another area. With the wide variation in availability of machine tools and tooling across the country there are bound to be differences in the interpretation of what constitutes a "fair" price. Here in the Midwest tools and tooling are still somewhat plentiful. As such the prices are somewhat lower than those in areas where they are more scarce. The average asking price for a used Bridgeport style mill is in the $2,500.00 range from most used equipment dealers. A 13" name brand lathe is in the same general price range. On the other hand I see asking prices of over $7,000.00 for for the same equipment from other areas of the country. When I see prices like that I have to ask myself, does the seller have an emotional attachment to the machine, have they done their research as to what the machine is actually worth, are the prices actually that high in the sellers area, or did they just pay to much?
When you're dealing with a sales model that spans the country you have to realize not everyone is willing to pay the same price for the same tool or piece of equipment. What passes as a "fair" price in one area looks to be outrageous and bordering on criminal in another.