I visited a number of them in my area years ago when I was trying to recover my stolen viola. Most of the merchandise on display appeared to be new stuff they would sell on an EZ payment plan with no credit check and substantial interest. Then there is stuff that people take in to sell to them. This is part of pawn shops' shady reputation because historically pawn shops were where thieves took stolen property to sell. That has changed a lot because goods acquired that way need to go into a property registry for a period of time (a week?) to be checked against police reports of stolen property. Then there is the stuff that is taken in as loan collateral. There is a time period during which it can be reclaimed by paying off the loan and interest. If the loan is not repaid, the item(s) become the property of the lender and can be sold.
I gather thieves were getting loans on stolen property that they then never reclaimed. That was the reason for my multiple visits looking for my viola. Of course the pawn shops hated to see me coming because if it turned out they had my instrument, they would need to turn it over to the police as stolen property and would be out anything they had paid to buy it or loans they had made to acquire it.
To a great extent eBay has replaced pawn shops for the more sophisticated burglar.