I've only bought an art commission once and it was from one of my favorite painters. I wont say his name since he works at a group home now and moved on from drawing loli guro pieces (although he probably still draws it privately), but basically I contacted this japanese artist I love since his english is pretty good, and offered him 100 dollars for a digital painting. I give him my paypal and other contact info, and he was flattered, but said he was too busy to draw what I had in mind, but told me he loved the idea. Half a year down the line, he sent me an email and informed me that he was going to send me an invoice. It was only 15 bucks, which I thought was strange, since I originally offered 100, but I figured "what the hell" and sent him the cash. A week or so later, in the mail arrived a physical copy of the drawing I had requested months ago.
From then on I vowed to never commission an artist again, since I will never have a better transaction than that.