Countierfeit battery sellers from Ebay and PPUSA

Is EBay sellers selling fake batteries or is it just me? I bought about three batteries and they have all died out permanently in less then one month. I even bought a $12 battery from PhonePartsUSA and it crapped out on me. Is it bad that i disconnect and reconnect the battery while having the iPhone still turned on or might have stressed out the battery cable to much while taking it out and putting it on to many times. What is there to do. No damage seems to be present on the batteries and i have been repairing phones for two years now.