Interest? haha, I am guessing not. Maybe a fee for putting them up in the store front. Like a closing sale eBay does. I don't know but that is odd. Do you know exactly how many things you sold at 1k since your last visit?
If you are from Canada..it's probably the GST that the government likes to take from us.
On another note, I went to the storefront yesterday and they had given me 200cr...not a lot you can buy with 200cr...maybe a free decal or something..