So are the ebay $30 upgrades mostly legit?
Hard to know. Personally I would not trust them. Probably some are legit, but the only way to tell is to have a SHA256 checksum of the install ISO (or disk), and compare it against a SHA256 sum of the original upgrade ISO. Without some sort of real verification like that, I wouldn't risk it. Frankly if I were upgrading now, I'd probably cough up the $120 (depending on version) for an official copy.
https://www.amazon.com/s?k=windows+upgrade+from+7+to+10
Your mileage may vary.