eBay doesn't make parts, there is no eBay brand. That being said, pretty much anything that doesn't have a real manufacturer, a known good manufacturer is a cheap knock off. If it says JDM in the ad, it's a dead giveaway that it's junk. Again, you get what you pay for. If you are finding one that is from some flashy seller, that gives no searchable name brand for $50, odds are its too good to be true. Look at the brand name, Google reviews for that name. There will be reviews. Especially the bad ones. A lot of the time, people don't really take the time out to post a review unless it is something that really pissed them off.
If you plan on boosting, don't waste your time on a header for NA, they aren't interchangeable.