The only used bike I've bought was my Raleigh Twenty, which, of course, was bought as a fixer upper. I did bid on a used Pocket Tourist on Ebay last year, but was outbid, and ended up buying a new one.
Generally, I'll buy a bike with a view to what it's going to be like while I own it, not what I can sell it for a few years down the road. My latest plan for my Dahon Speed is to do a drop bar conversion. If it works out, it should be pretty nice, but if I wanted to sell it, I doubt if I'd get more than maybe $150 for it on Ebay, maybe not that. Why should I? It will be used, with used components, as spambait points out (new price was $420 + tax from a local shop by the way). I don't expect to sell the Fridays for years, if ever; they do tend to have a high resale price, but by then I wouldn't get my hopes too high.
It's nice to dream of selling your old bikes at a high price, but the prospective buyers are dreaming of finding one at a low price. Just enjoy the bike while you have it, and try to make it last.