Car wont start and no clue

So I bought a used headlight off eBay and was too scared to do it my self don't know why. I had some friends come and do it for me. I was at my grandmas visiting and my kids were in and out of the car, when I left I got in started it, and as I think about it the past couple times when I started it it took longer to start or 2 times turning the key to start but nothing big cause I never payed attention. If it was serious I would have definitely noticed. But I got in it took a little extra to start and I went home parked it and 10 minutes later my friends come. The eBay thing said make sure the battery is disconnected or if it doesn't work or fit they can't take it back or whatever, my friends tried to disconnect it but didn't have a good enough tool. They even beat the battery thing to try and knock it loose but it wouldn't so they just did the headlight without disconnecting it. When I went to see if the headlight worked I turned the lights on and they said no, so I turned the key and they said no, so I tried to start it and there was nothing. Usually it dings and all that but I couldn't get the windows to go down or the locks to work. They said my battery was dead and asked for jumper cables and they tried for 30 minutes to jump it. It didn't do anything and they left. So the next day I finally found a battery charger and hooked it up took all day charging and the charger finally said ful and the voltage was at 12.5 so I tried it, it made a noise and nothing after that. So I stopped a minute and just turned the key to check everything else. The lights, windows, locks, radio and everything worked, even the lights on the dash and the ding was back so I tried to start it and it made a noise and everything turned off and then no noise after that. So I found somebody else local to come try what they could. They tried something to do with the chip in the key (don't know if they did it right) then tried to jump it it made a tiny bit more noise, then he disconnected the battery and stuck the things in the jumper cable things into the battery and tried jumping it, hooked both the battery charger and his truck to the battery all that, that didn't work so he taped on the started to see if it would do that thing and it didn't work. I noticed really light fluid on the driveway that I had seen a couple times but thought it was from the neighborhood cat. It wasn't pure black but kinda dark liquid. A couple weeks ago I took it to walmart for an oil change and they put a little paper in my window saying that my battery didn't meet standards or whatever and I should replace it then all this happened, the only issue Ive had with this car since I got it about 3 months ago is rusty fuel and needing a fuel filter and its weird how there were no problems before walmart had it, anyways I've been told I could have a bad battery but every time I check with the charger on the voltage it always say above 12 volts even after trying to start it and having everything on. Everything always comes back on even after it all turns off when trying to start it. It takes a few seconds but everything always comes on so I don't think its the battery. Some people suggested starter and some people say my alternator could have went out and I drove home from my grandmas on pure battery and that's why it was drained, I need to know what it is cause I can't afford exploring the issue. They said, start by buying a new battery and if that isn't it try the alternator and if that isn't it try the starter, I don't got the money for all that! I need to know what it is so I can take it to the garage and order my own part! HELP PLEASE!