Interesting that prices came down so fast. Just a couple weeks ago the BIN prices were $650.
Personally I have no problems buying a phone on Ebay - probably 8 of the last 9 phones I've owned were Ebay purchases. But in this case, as it is your primary concern, I would ask the seller to take detailed/hi-res photos of the screen to show no dead or stuck pixels. However, a seller may not be willing to do that if they still have the protective plastic on the screen (they might take the photo, but they'd be crazy to take the plastic off for an Ebay buyer with a pre-sales question). Still, it's worth a shot. If you find a seller willing to work with you on this, that's a good sign by itself. Maybe you find a lightly-used one where the seller can accommodate this request. It's in their interest; if they happen to have a perfect screen the value of the device goes up for some buyers.
A good seller will disclose something like a stuck/dead pixel (I would), just like a scratch on the screen, though if they are really selling NIB then in theory they haven't even opened the box, or if they have the phone should not even be touched beyond verifying that it is in fact in the box.
So they might not know. Bottom line, as with any Ebay purchase, look for a seller with good feedback who appears to know what they are doing.
I usually wait 6 months to a year on the Ebay thing, but it really depends on what prices do and how much I want the phone. Prices will always go down over time, but with these ultra-high-end phones it will be a gradual drop. The TP1 was about $300-350 when I got mine (I think about 6 months ago? maybe a bit more) and now they're in the $225-250 range, give or take. As great as the TP2 is according to many users, it's not exactly the death knell for the TP1, so the latter is going to hold its own for awhile. Don't forget that the Sprint TP2 hasn't even hit stores yet so there's just not that much inventory out there to cause a price drop.
By the way, even if you have insurance, if the stuck pixel thing becomes a real problem, they might start doing what laptop/LCD monitor mfr's do, and that is to specify some minimum number of "acceptable" dead pixels before granting a warranty replacement. I've read unofficial numbers in the 4-5 pixel range, though there are some rules about proximity as well. With an 800x480 display, the number should be lower since it's a smaller screen, but still I wouldn't be surprised if eventually they start rejecting insurance claims over 1-2 pixels. All the more reason for you to find a perfect one out of the box, I guess.