If you sold the car, and the leak just started with the new owner, I'm betting he is adding fuel after it shuts off (rounding up to the next even $$ amount on the pump), then parking the car in the sun. The tank is getting over filled.

I live a block from the gas station I use here. I have learned not to fill the car then drive it home and park it. I fill up before I'm leaving to go somewhere, and then hang up the pump when it shuts off. I'll pay whatever the odd change the pump may shut off at. Gene