I am very uncomfortable buying any type of computer parts from people on Ebay, especially used ones. This is a disappointment waiting to happen. Your choice though, after all, it's your money.
I'd forget the Celeron for gaming, it just dosen't have the muscle for it. Celerons are for casual computer users who don't get into gaming like a lot of us here on TF. The P4 is a different story though, if it's in decent condition, it'll make a decent brain for a gaming rig.
Anyway, neither of the sellers you are dealing with are Powersellers, and in terms of selling on Ebay, they're both newbies, less than 1000 unique auctions completed. If a seller dosen't have that Powerseller tag, I'm not bidding for much over $20 on anything, it's just too risky. Anyway, that's just me, I hope it works out and you get a good deal.
I\'m already visualizing the duct tape over your mouth...