Magic: The Gathering card prices?

Dear Brain Trust,

I played a lot of Magic: The Gathering when the game was new, and through the first several expansion sets, before eventually drifting away for various reasons. At one point I sold a few valuable cards individually on eBay, and gave most of the rest away to young friends who were just getting into the game. I held back a few cards that I had a nagging feeling were or would be valuable, or that I just had sentimental attachment to, and that weren't going to make a difference to my friends anyway.

I got email from Origins (a gaming convention we'll be attending next month) that, among things, highlighted a dealer specializing in collectible card games (CCGs) who will have buyers at the con -- so, the email says, bring your cards if you're interested in selling, either individual cards or collections.

So hey, I said to myself, what are these cards actually worth? I looked up some of them on that dealer's site -- that is, what they are currently selling these cards for -- and my jaw dropped a little. But that's sale pricing.

What is a typical range for the difference between buying and selling prices? What should one reasonably expect a dealer to pay, as a fraction of the selling price?

I would have thought this would be something I could answer with a web search, but either it's not or, more likely, I'm not formulating my queries well, this not being the sort of thing I generally do.

Anybody have any advice that will help me evaluate price offers from a dealer?

(I know about grading as a concept, but I think that's orthogonal. Dealers sell cards that are near-mint and cards that are well-played and everything in between. The buy/sell ratios would be about the same across the board, wouldn't they?)