here's what's happened over the last 10+ years or so of cards:
new set comes out. cards immediately get marked up the few weeks before beckett comes out (used to be before the monthly came out; now it's before the online part of the monthly comes out). beckett prices things. everyone then crashes prices at "half beckett" except on the super hot card, or if (during season) someone takes off unexpectedly.
if you're a collector who wants the basic sets without putting things together, you've got a great deal. many people will blowout their sets on ebay and other places, at a loss...why? because they need to get the cards and hope the special cards, inserts, etc. get them enough profit.
it's near impossible to make money off cards now unless you're basically playing quick-turnover middleman. there's no reason to keep cards around longer, as they retain little of their value.
(and as uptik said, graded cards are where it's 'at'. and that's on the hot or speculative items, in super pristine condition).
oh, one other thing. i got into cards in '75, did some flea markets and shows from the late 70s - mid/late 80s, and stopped collecting (the second time) a couple years ago, though i still keep up with things. anyhow...what was considered "MT" or "EX-MT" then is likely considered EX or worse today. the dogbeat mickey mantle cards still retain some value, and the wonderful condition mickey mantle cards retain it. but the inbetween condition ones - enough that it looks nice, but not pristine - there's a significant difference between what you could have gotten (% of topline) 15-20 years ago and today. those middle grade cards have a lot less demand...