Q: After sitting for months, jump-started battery

"My pickup hasn’t been driven in several months (and I’m trying to sell it). The past month the battery was dead and I’ve jump-started it a couple of times and have let it charge for the past 2 weeks. It will start a couple of times, but if I’ve let it sit for more than overnight, it is completely dead. So much so that I have to unlock the door with the key and no lights turn on inside. When I try to start it, nothing at all happens. Almost like there’s no battery installed at all. Before all of this when it was a running pickup, there were several warning lights on that weren’t on when I’d stop driving, which were: ABS, two VSA lights, emergency brake (which I know was not engaged), and tire pressure. The battery and check engine lights are not on (which should be on). Also, after charging it and turning it off and on several times, I used the command start to try to turn it on in order to make sure that it was still working. It did work, so I turned it off and locked it up. Around 20 minutes later, the alarm went off and the only way to get it to stop was to disconnect the battery. I tried to reset the computer by disconnecting the battery for 15 minutes and then reconnecting it -- but no success. I have an appointment at the shop in a few days, but first I would like to know your diagnosis and what should be done?"