Formal:
* Loan office
* Collateral loan company
* Pawnbroker (this is the person who runs the pawnshop)
Informal:
* Secondhand shop
* Thrift store (while these are not technically pawnshops, they often sell items that were pawned and not redeemed)
* Flea market (similar to thrift stores, but with more variety and often a focus on antique items)
The best synonym will depend on the specific context of your writing.