If you don't know all about the item and worth, you can pay handsomely. To a pawn shop, all items for sale are a chance to make money. From my experiences, here are some tips. This is backwards from the kind of high-volume sales that most retail works on. Pawns know that they will at best have a couple of similar items, so they are targeted at highest dollar per item, not cutting price to increase volume.
Pawns DO know a good guess at the used price of stuff.
They are in the business of getting good deals for themselves. They accumulate data on used sales prices of lots of stuff, particularly vintage or collectible items. If there is a printed guide to the sales prices of some set of stuff, you can bet that the regional or national pawn chain store has a copy under the counter or in the back. It is worth money to them to know this information. Local pawns may or may not be as well informed.