Re: looking for 7 string tremolo equiped Ibanez
If you buy a used 7 string for market value, you should be able to sell it for market value (as long as you don't ding it up, etc.). It's as simple as that, as long as the market value doesn't change significantly while you own it. Since the 7-string market is low right now, I don't see how it can get much lower.
Of course, you'll have to do the research to determine what the market value of the 7-string you want is. I recommend checking e-bay regularly, and looking at the price that that model sold for ("completed auctions").
If the guitar salesman was talking about buying a new guitar, then I'd probably agree with him. As soon as you buy a new guitar (6- or 7-string), it's instantly devalued by at least 20% or so.