Take it easy yuleh, you arent guilty for anything.
She gave you a price, you bought it. Its her fault she gave such a low price.
But if you were a nice man, and you seem like you are [As you seem guilty and upset about how you got enough firearms to start a gang for a cheap price], you should do something kind for her.
You fixed her car, free of charge. Thats good, but I doubt it would have amounted to the original price of all the crap you got.
If I were you, id get her a not-really-expensive gift. Id get a really fine set of cutlery, or maybe a REALLY
nice painting. It will make you feel better with yourself, trust me. Ive been through a similar path [But with a lack of guns
If you do get her something, though, she might think your trying to hook up with her [Mad world, innit?]. So you should make up a reason for getting her a present. Say something like "I just checked on the internet and the Ruger I bought from you is actually much cheaper than it should be, so I thought id make up for the amount with this gift...", or maybe "Ive been going through my garage and I found this very nice painting. I was thinking about what to do with it when I realised that it would look absolutely wonderful with the wallpaper in your living room, dont you think so aswell?'
Something along those lines.