Vintage prices may go down a little, but they will still be up there in the 100's.
Reasons I feel this will happen:
Currently there is/was a massive demand for old Star Wars figures in the early to mid 90's. The real fans like us who want to have missed childhood items for our own private collections are going nuts hunting and buying up old things. We have spured the current lee's price through the roof with our "need to have it" attitudes and "no price is too great" thinking. And once we have these items in our possession we probably aren't going to re-sell them anytime soon, unless we hit a major financial crisis.
Once flash in the pan collectors fade off, the quick buck folks who are buying and selling just to make a fast buck or two are gone, and the demand cools off a little like it did in the mid 80's the prices will still be high, but may come down a little bit. The "rare" or mint items we all are currently looking and hunting for will all be in private collections and most of those hard core collectors will be unwilling to sell off memories unless the price is right...which is usually higher than listed price. The demand might be less than it is now, but there are always collectors out there looking, and there will be people getting into this hobby after Ep III or beyond.
I remember going with my mom to all those antique doll shows in the 80's and early 90's, sitting in the booth or just wandering around and there were a lot of old star wars items floating around and I remember that even then the prices were high. I wanted a 12" boba fett so bad, but even then collectors wanted 75 dollars or 50 for one in a box, and 40 for a loose and in good shape. And this was in the time where the demand was little to non existant.
As for Ebay messing with or being the future base for pricing is a stupid idea. Ebay is nothing more than an on line auction house, not a dealership or shop that mainly deals in toys or the toy industry. Auctions are not a good judge to find a mean price for anything.
As with any auction, if you know the collector value, you can get a good deal if others don't know the collector value, and you can get hosed if other collectors know the going rate and have deep pockets and just want it. Ebay is nothing more than a crap shoot, and sometimes a game of luck.
Besides no one could accuratly find a middle ground pricing with all the various grades of items floating in an out of the system. There no standards for grading and classifying collectable items. I've bought items on line that were claimed to be c-8 or c-9, but in reality they should have been c-6 or c-5. The images of the items didn't show the extent of the damage.
I've see incomplete damaged star wars, transformers, or any other hot collectable going for more than what it should. I could say the same for obscure architecture books I've had an interest in buying. Where I've been the only one bidding and then out of the blue on the last day 5 other people want it and the price is 5 times more than what it normally should be. There's no way to know if there isn't price setting or inflating by the seller. They can jack the prices by having two differnt computers with differnt IP address...home and work...or friends bid the price higher to lure you into paying extra. There's no good way on Ebay to grade or value items. If we did us Ebay as a standard, we'd all be paying more for junk.
Ebay is fly by night folks looking to make big bucks or some thing of a profit off of junk and soak us collectors. I still scratch my head when I listen to folks bellyache about paying too much for gotta have items. At least with comic shops or toy dealers, Lee's is a benchmark dealers use to stay constant since its an average price from the various shows. If a comic shop get in a vintage figure in a box or loose, he checks the prices and adjust accordingly to the wear and tear on it. Rarely do they go above lee's listed price unless its perfect in every way, and a good collector will know his pieces and about running price.
I don't think the vintage star wars collectables will come down greatly in price. The hard core collectors will keep the prices up and keep using lee's as a reference. I have bought and found better deals at antique toy shows and comic shops than I have on ebay because Lee's is a collectors industry standard. Not to mention I have my lot (vintage and new) insured with some of the dolls my mom gave to me for safe keeping and storage, and my insurance company wants a printed industry source when I submit my information, photos and written listing. If I was to submit and Ebay listing, they would probably laugh at me.