If I were me (and I kinda am most of the time) and looking for that specific card, I'd pay $150. If I were someone else (which takes up the other times I'm not me) and just thinking it's an uber upgrade, I'd pay $225 installed OR shipped.
If he/she doesn't shop around and has the ideal that the 285 is the answer to a system that ails, you can probably get more out of them. But he may be a friend that may have a lot more questions than you're not ready to give at that unknown price. If it's really a good friend, I'd cut 'em a deal before putting it on eBay. You've got a card, it's just laying around, you can get money for it or help a friend out rather than have them look like newegg reviewers left in the dirt. Since I'd pay $150 for it myself, I'd say that's the number I'd give to a friend. Be firm though. If he haggles, raise it $5 at a time because it's already a deal. "It was $150, but $155 now sounds better...I can get $200 right now for it online. Oh? $160". Hopefully they stop there and just take it.
Checking: $133 w/o shipping, 8 bids, and 3hr left on eBay now. Buy it nows are a bit much as expected...so I feel comfortable with this. $150, install it, see it run, then wipe your hands like Pontius Pilate. Just made your GTX580 $410...heck of a deal.