optima killed in its prime

So I've got plenty of trouble shooting I have to do on my car but I wanted to start with this question and see if I can get more than one answer. My car's been having problems starting - and I thought I had it narrowed down to the battery so I bought a new Optima Red Top. Thing is I didn't technically buy it new I bought it off craigslist due to - well ya know the deal. Anyway I check the voltage on the seemingly new battery when I get there and sure nuff it read 12.7v or so. I'm thinking it's all good - so I install and basically my car was having issues running - and let's say in a day or two I come back to check the battery and it's at 4.6v or something. I brought it to pep boys and they tested it - came out bad and unable to hold a charge. My question - is it possible I was done in somehow by a scam. Could I have killed it by not charging it correctly on its first run. Or could something in my car maybe have done it?

My money is on a bad battery.

I agree the battery is bad. It would only be damaged of your car goes through irregular charging cycles** and extreme temperatures... ** I had this happened before. The alternator would only charge at about 900rpms. The car idled at 600RPMs. SO my guess was that the constant fluctuation of start stop of every day driving ruin the battery. However they were Interstate batteries not OPTIMA, but I'm guessing it could happened to OPTIMA too.