Surprisingly, ones from eBay hold up pretty well. I have 3 and have not had an issue with any of them. If you’re testing any in person, like at the shopping centre stands, turn them up and press them with some force against yourself. If the motor chokes a little, that means the quality isn’t quite there.

But most friends I’ve known who’ve bought various brands have not had any issues with ones up to $100. Also just be mindful of where you press. Hitting areas of inflammation (burning/electrical feeling) can aggravate the area.

Edit: Also, make sure you have a good knee strengthening routine. A lot of good free information out there.