Things you should never buy online (and it's not obvious that you shouldn't):
Button batteries (CR2032, etc). They're almost always almost expired or have no date code (ie. factory rejects) - Tudor Bosman
Cheap iPhone cables/chargers -- cheap for a reason, I've found. - Stephen Mack
Odd Tudor, we buy a lot of CR2032s online and have rarely had a problem with them. - Rachel Lea Fox