How much would a used 7990 sell for?

~2000 hours of use, probably 1500 of them for gaming and the rest for assignments. I live in Australia, where new ones are selling for $1,000 on eBay and stuff, not sure about prices on other forums though.

Just cut those prices by 20% if you want to get rid of it. though something like 60% of those prices is probably what someone's going to want to pay. Either way that card is going to be a rather tough sell, best route would be to find some family or a friend who wants to get into PC gaming and give them a decent deal on it.

So convince my friend that it's worth $1,500 and is some kind of super GPU that will last for 15 years you're saying?

maybe around $350 ~ maybe.

less than $250 or $400-$500 or $800.. These are sold listings on ebay.au
Same card a few times though so make of that what you will, Not many buying these cards in the oz or am I pressing the wrong buttons??
http://www.ebay.com.au/sch/i.html?_from=R40&_sacat=0&LH_Sold=1&_udlo=&_udhi=&_samilow=&_samihi=&_sadis=15&_stpos=&_dmd=1&_ipg=50&LH_Complete=1&_nkw=hd+7990&_sop=2

Hey ! out of curiosity you are replacing by what ?
The 7990, depending on the model, still rocks; besides a few new toys that are not fully functional, direct X 12, and, ok considering the fact that the 7990 is a power guzzler and an effective heating unit for winter (lol) (by the way, with proper setup these factors are easily managed, I cannot find a really worthwhile upgrade. Something that would really improve visual experience and immersion (besides 3D) does not seem to appear for me.
You ?
I like the DDR5 AX7990 6GBD5-2DHJ (huge monster, and must have nuclear power plant close-by.