I have not had any firsthand experience with open boxed items from any retailer. However, I have had a couple of friends and family members buy opened boxed items from various retailers. Most of them had very good luck and everything bought worked good with no trouble. Others did not. The thing you have to watch out for is the Warranty and the RMA/Return policy for open boxed items. Every retailer has different policies toward open boxed items, and before you buy anything from one you should consider reading it.
Other than that, I would try to go for a GeForce GTX 660 or 670 rather than the AMD card. But that's my preference. Either way, that is still a good price for that particular card.
I have recieved 4 motherboards open box from newegg. I have never recieved the IO shield or the correct sli/crossfire adapters when I order open box. Sata cables are often missing as well. The one time I got a graphics card open box it came without the driver disc and the dvi to hdmi or vga adapters. When ordering open box just assume you will only recieve the most valuable item in the box. Everything else could be missing. Hope it helps.
I dont mind much if everything is missing besides the card itself. I'm actually running on an open box card, But it was only $75 so it wasnt to much of a risk. Is it a probable chance that the 7870 tahiti open box will work?
I'm guessing that there's a higher chance that it works, as most open-box items IMO do. But there's also the small chance that it is one of the oddball open-box items that doesn't work. I say trust your gut