The short version is yes, people often go to a pawnshop intending to sell, rather than pawn, an item. This doesn't mean that the definition of 'pawn' is incomplete, though. To understand how this situation would arise, think about how pawnshops work:
When someone pawns an item, they are borrowing money using the item as security (exactly as the definition says). For a specified span of time, they can bring back the money (plus interest) and get the item back. However, frequently, people who pawn items are unable to come up with the money during that time, at which point the pawnbroker will offer the item for sale to recover the lent funds.
Pawn shops, as a result, become a place where a wide variety of items are available for purchase. Some of those items are more valuable than their owners (or even the pawnbroker) will realize, and so antiques dealers and other experts in obscure goods visit them, hoping to spot a valuable item available for cheap.
As a result of this, the pawnbroker will often be willing to straight-up purchase items, in hopes of making a profit selling them to those traveling dealers, and so people will come to them not only when they need a loan, but when they just want to sell something.
Many businesses are generally understood to offer services that are not, strictly speaking, part of their formal definition. For instance, gas stations almost always have a machine for re-inflating tires. Most pharmacies sell a variety of general household goods in addition to filling medical prescriptions. Banks offer safe-deposit boxes. When enough people who want one service would also want another, and it is convenient to provide it, businesses adapt.