Joss. Ebay already has the type of venue youre talking about. Sellers have the ability to offer an item through their "Ebay Store", which allows the seller to showcase items for extended periods of time for VERY little money, and at fixed prices. The ONLY real drawback, unless they have changed it recently, is that these store items do NOT come up when you do a regular word search for a particular item. I believe its still the case that only auction items come up in word searches and fixed price store items do not, which obviously, drastically limits your exposure. If Ebay would allow fixed price store items to come up along with the regular auction listings in a regular word search, i would definitely open a store, actually, i already have one, but never use it for the reason stated above.
Imagine how nice it would be to go to Ebay, do a simple search for ie. Jerry Corbit, and get a list of 10, 20, 30... Corbits currently for sale from several different purveyors/collectors. Throw in easy Ebay or PayPal payment options, good pictures/descriptions etc and youd have a hell of a market force.
Ebay of course has good reasons to prefer auctions over fixed price store listings, they make more money with auctions since auctions sell the item at a much faster rate than long-term, fixed price store listings. Seems to me, to make store listings worth their while, theyd have to increase the final value fee a bit, so youd end up paying a higher percentage of the final sale value of a store item than an auction item, and still, it might be worth it, depends on how greedy Ebay is.