From personal experience, some have to be factory seconds, some are grey market importers, and some are simply reselling exactly what is on sale at Nashbar ( apparently the clearinghouse for Performance Bike).
Not a few folks sels a lot of Tufo stuff on there but are not an authorized dealer so they must be importing it from someplace else and just undercutting authorized dealers. The tires are perfectly OK otherwise. Then several folks get the same el cheapo tires that Nashbar has but sell them for the same or more, whatever those are this month. Some folks sell tires that are not sold through any other reputable bike dealer in this country. For one case a type of Michelin tires that are were no longer available. I thought I’d try it and these were hands down the worst bargains I ever got. The rubber was so soft I ended up with so many cuts the tires were unuseable after one week, but too late to get any justice on EBay.
The cheapest way to get reputable tires is joining a bike or tri club that is sponsored by your favorite tire company.
I have bought many tires off of EBAY and I have never experienced any problems. If they are in the original box/wrapper, I have a high level of confidence. They may be seconds, but they have performed as well as any I have paid full price for in a shop.