You could always try replacing the Consignment Store cash register or the store itself. Of course, if you're doing a challenge, you'll have to check with your challenge coordinator because the first method is using cheats. So here we go.
1. - First turn on testing cheats (Ctrl+Shift+C => testingcheatsenabled true). Then, Shift+Click on the ground of the Consignment Store and select either Build or Buy on This Lot. Afterward, find the cash register (I believe it's in the Build mode, under Community Lot objects or something alike) and replace the original one with it. Go back to Live Mode. Soon a new clerk will arrive and (hopefully) you'll be able to sell your items.
2. - The second way is used rarely because the first way usually helps, but if it doesn't, you can try replacing the Consignment Store itself. Go to Edit Town mode, find the store in the Community Lot bin. Bulldoze the original Store, and place the new one.
If neither of the methods helped, then I guess your save file is bugged. Best of luck.