I own about 200 tubes and a modern tube tester. I've bought on eBay, but I'd be very careful buying there having had both good and bad luck with sellers. This is especially true with NOS, but you can also buy substandard current production tubes. Unless you own a good tube tester, you have no idea what you are buying, so you must pick someone you trust.
One major problem is that many eBay resellers use vintage emission type tube testers that don't test tubes for transconductance or test at a high enough voltage that the tube will see in a circuit. Even they don't really know what they are selling!
Another problem is manufacturing tolerances are all over the lot for current production and a lot of new tubes that test below spec still get sold as "New" rather than rejected for QC. Even the very reliable sellers mentioned above have to weed this junk out if they want to maintain their good reputation. If you pay a little more, believe me, they're worth it.
Good luck!
One major problem is that many eBay resellers use vintage emission type tube testers that don't test tubes for transconductance or test at a high enough voltage that the tube will see in a circuit. Even they don't really know what they are selling!
Another problem is manufacturing tolerances are all over the lot for current production and a lot of new tubes that test below spec still get sold as "New" rather than rejected for QC. Even the very reliable sellers mentioned above have to weed this junk out if they want to maintain their good reputation. If you pay a little more, believe me, they're worth it.
Good luck!