I'm actually not seeing that. Maybe I haven't looked long enough but the only used gear I'm finding is just slightly less than new (within 10-15%) and often times as much or more. Maybe I just haven't looked in the right places but fleabay's prices are crazy. For anything within 30% of the new cost, I say buy it new with a warranty. That way you know what you're getting. Also depends on total cost. Less than $100 it probably doesn't matter as much but if spending $200+ I would absolutely buy new if the used prices are within 30% or less of the new cost.
This is highly subjective, depends greatly on what brand and model you're looking at.
Something like a GTX 780 that was $500 when it was released is now under $200 (with $130-180 being fairly common) for a used card. GTX 680's that were $500 at release can now be had for less than $150 oftentimes. A Radeon HD 7950 that was $450 at launch can now be found for $65-200 (depending upon whether it's the 1GB version or the 2GB version).
GTX 970's that were $300-400 when released I'm now seeing going for $250 and under, with $155-220 being fairly common.
I do often stand behind the notion of getting more value (and/or performance) for your money by buying used (plus you're recycling old parts at the same time), this is especially true if you're on a limited budget since brand new cards that are really cheap typically don't perform well. Buying used can be a good way for the budget conscious buyer to save some money on upgrade fees or fees for a new build, though there is some question as to what parts are "okay" or "safe" to buy used.
This saying applies here: What's cheap isn't always good, and what's good usually isn't cheap. Really cheap, new graphics cards pretty much always perform like junk/rubbish. They're low performance cards that are basically just suitable for basic 2D office usage, with 3D gaming responsibilities (at reasonable frame rates and resolutions) being out of reach, and are usually overpriced for what little performance they do offer.
I'm going to guess you were probably looking at newer cards from current model lines from AMD and/or nVidia. In which case you would be correct, in that current gen cards typically don't lose much on the value/asking price when sold as used or pre-owned. I've found that you have to look 1-3 generations back (or more, depending upon budget constraints) to see what we mean by getting greater value for your dollar/more for your money.
