What will be the lawsuit?
No store is required to accept returns.
Costco explicitly states you can return any item you want, with restrictions. Those restrictions include electronic, diamonds, alcohol, cigarettes, and the like. Nowhere is it implied that things like toilet paper cannot be returned, and even if that's the policy now, it wasn't in place at the time of the panic purchases.
I am not trying to defend the people who overbought supplies at Costco and elsewhere (to the detriment of others). I am just pointing out what the lawyers are going to harp on if Costco prevents the return of these items purchased before any policy changes, if they indeed have been made corporate wide.