I had this problem to and I did 2 things to fix it. I'm not sure if its one or both that fixed the problem but this is what I did:
1. cancel the sale items that have negative days on them, and if that does not work or you never sold anything...
2. Reset the seller using "resetsim (firstname) (lasname)" I think you need testing cheats enabled first to do this. Then the seller will disappear (back to his home lot)and for a couple sim hours the store will be empty until a new seller arrives who is glitch free and happy to take your Simolians.
