One thing I didn't see mentioned by others is the potential liability you incur when you spin wrenches for someone else for money.
I'm not a lawyer, but I think that if you accept money for a job, then if anything happens to the truck and causes an accident or worse, you may be liable, no matter what verbal agreements you made ahead of time. So I'd want to have insurance and/or an ironclad release form.
Again, I am not a lawyer, and for all I know, you could face similar liability if you do repairs for someone as a favor, but I expect you open yourself up to more risk by accepting payment, as now it becomes a business of sorts.
I know that the majority of people would never come after you if you make a mistake, but things change very quickly if you have a bad day and forget to fully tighten some critical nut which causes an accident or injury.
For that reason I've never done repairs for people for money, only as a favor.
I had a somewhat similar experience as you when I frequented the local junkyards and started taking "orders" from forum members for things they might need. I usually made a few bucks on each item, but really after all the hassle of making contacts, looking for the items, pulling them, packing and shipping, it made no sense. I don't need the money, and I was doing it for fun and to help out the community. But after countless cases of people who asked for something then never paid for it, or just disappeared, I stopped doing this. The only good thing to come out of the experience was that I came across some really cool people like [MENTION=148218]JZiggy[/MENTION] and [MENTION=139000]phattyduck[/MENTION]. That definitely made it all worth it!