My trade-in to GS tonight:

Chaotic Shadow Warriors - $5

Darksiders - $8

Big Brain Academy - $6

Top Spin 3 - $4

Brave: A Warrior's Tale - $2.50
Yeah, so on the surface, it doesn't look great: $25.50 total, or only $20.40 if I took cash. Though, really, even that isn't so horrible.
But because of the Edge card and trade 5/$20 bonus, I ended up with $48.05 in credit.
At
best, on eBay I'm going to get $20 shipped for Darksiders (or even as little as $15), $10 shipped for Chaotic (with some auctions ending as low as $7), $10 shipped for Brave (and that's for new copies, since mine was new

), $15 shipped for Big Brain Academy, and $10 shipped, if I'm lucky, for Top Spin 3.
So that's a total of $65. But wait, I said
shipped, so I have to deduct shipping at around $2.50 each (that is using very cheap bubble mailers bought online), plus, oh yeah. eBay fees of 15%, and PayPal fees of around 4%. So the total take after all that is... around $40...
Now, sure, $40 cash is probably better than $48 at GS, though I will make use of the GS credit eventually. But you also have to factor in the hassle of packaging and mailing 5 different things and the potential for problem buyers and bad feedback even when you do everything right, etc... I do sell on ebay so I'm not one of those people that dismisses it as a big headache, but sometimes (more often that you might think), GS can be the better option.
Clearly, the cash option at GS is what is for the most part pure stupidity. You are taking 20% less and losing out on the promo deal. If that were the only option, I'd definitely eBay the stuff for twice as much even with the hassles that entails. So, in the end, unless you are unloading stolen goods, I don't see why
anyone takes cash from GS. If you are a kid or parent looking to dump a bunch of old games to get something new, they you ought gladly to take the credit + bonus promos and buy your hot new game with that. But it astounds me how many times I see kids/parents taking cash. I think part of the problem is that most GS clerks aren't aware of the promos and don't total it up to the end to figure what the cash vs credit difference really comes down to. For example, in my trade above, the clerk would probably say "$25 credit or $20 cash" and most people, even if they might want to buy something at GS later, would take the cash. But the promos usually factor in when they are finalizing the trade so neither they nor the customer sees that at first.