NEVER buy universal poppets unless you know what you are getting. Buy from a reputable source.
I bought a couple of universal poppets from my lhbs a while back and have been happy with their performance. I saw a deal on Homebrewfinds for packs of 5 @ $10. Figured can't be much difference and I didn't fully remember what the poppets from the lhbs looked like as they were in the keg. The poppets were delivered and they are complete garbage. The wire is thin and I can easily see where the spring favors flexing verses compression. I ended up buying more from the lhbs for $3 each. Pics below.
Junk on the left. Notice how it is already bent. That is how they were delivered. They don't even stand on their own.
Junk compressed. Note how it naturally wants to flex out when compressed. This causes the stopper to get wedged in preventing a seal.
Quality poppet. Compresses down without flex.