At my early cons, I used to do spot watercolors on index cards while waiting for customers, and I would sell those watercolors for $1-2. I got the idea from Angela Boyle, one of my early table mates, who makes much of her con sales off of similar illustrations.
I had just finished an illustration and put it out on display. Someone came by, decided to buy it, and picked it up. They also picked up a copy of Wendigo and flipped through it to see if they wanted it. Unfortunately, it turned out that the watercolor wasn't dry yet, and it left a big yellow smear on the back of the comic. The customer, not realizing that the comic had been marred, put it back, bought the watercolor, and left. Leaving me with an unsalable copy of my comic, something my dad would call, "Tuition in the school of life."
The moral of the story? Keep your illustrations behind the table until they're dry.