My biggest worry is about buying fakes which may be either useless, or worse still, dangerous, e.g. phone or laptop batteries. I don't necessarily go for the cheapest ("you get what you pay for" as they say). I read numerous reviews, although I know there can be fake reviews.
My first mobile phone replacement battery was described as "genuine" - which it may have been, but definitely not new - probably salvaged from a traded in phone: came wrapped in a small piece of bubble wrap, no original packaging or instructions. It worked only marginally better than the one I was replacing and died after 6 months. The next one I bought was a Chinese replica: paid a bit more: works well, doesn't overheat.
Lap-top battery: got one that was twice as much as the cheapest available, but had excellent reviews, and half the price of one from the computer manufacturer. And it was good: arrived already charged, lasted longer per charge than the original, doesn't overheat either.
But now, after 3 years of use, getting towards the short charge scenario, so looking again for one as good as that one was (but can't find same again.)