Take a welding class.

Then, go to a welding supply store and price out new welders, often they have some pretty good deals and if something goes wrong, you know they will back it up. Ask them if they have any used welders on hand. The store I deal with takes old machines in on trade when someone buys a new machine. The name brand machines, they check over to be sure they function correctly, and generally will sell them at about 2/3 of the price of a new welder of the same equivalent.

The only way I'm buying a used welder off Craigs list is if I can spend some time laying down some beads, and then it better be dirt cheap and a name brand. Then I'd still be nervous. Gene