Arguing my intelligence

For a while I fixed iPhones because it was fun. Then I started getting people questioning my work and being petty. So I quit doing it because people ruined it for me.
There are still a few people who come to me for help and im happy to do the work because I like the people and still enjoy fixing phones and the process of troubleshooting.
Yesterday I had a guy drop off 3 phones. Sweet, lots of fun. Only problem is that he is selling them and trying to make money from them and in the process im getting screwed. not only that but he is questioning my processes and abilities. he wanted to take one of them to get the software changed by someone else because I told him that is was not possible right now to do what he wanted. I KNOW this. Its pare of what I do everyday as a card carrying geek. I know what models of phones can use what software and what the iPhone Dev Team is working on, and what GeoHot is working on and what MuscleNerd is working on. I keep up with it because it interests me.
The thing that annoys me the most, is that I become convinced that im wrong, that I just don't know what im talking about. so I question myself and what I know and start to feel like an idiot.
Well, turns out this other guy who says he can do what I know is not possible(right now) was just trying to steal my... client? Customer? Associate? bud? 's phone.
Its like in college when 30 people in my class all were telling me I was gay, and I started to believe them and question myself (Yeah, that happened). I guess I just have a hard time believing in myself and I hate that.

So there's that.

Yeah, still depressed. Its not going well. Hasn't been this bad since college. Thanks for the comments trying to help. I see it and appreciate it. I just might not say so.
For some reason this time, I also am having some serious social anxiety. Yeah I know! Me! I dont want to be around people. What the Hell!