Generally, a surplus shop is one that sells either secondary acquired items from larger department stores/online stores like Amazon that are in excess or sells items bought from customers similar to how a pawn shop operates.  They can sell a range of items that fit into multiple categories.  A good high-end example of that is the US department chain Marshall's (although they are a national department store chain, they are considered a "surplus store" since they obtain their item inventory from 1st sellers like Walmart).

I looked through the tag list and it might be worth adding "shop=surplus" just to cover these types of stores.