Garages generally don't buy from private sellers, too much risk. Good salvage yards that offer warranties and good parts are known to those in the biz.

The OP said the $ 2500 quote was replace old engine with used one so that would include engine, misc parts and labor. Sure, there are less expensive ways to make the car go again if a person does their own work, sniffs around for parts. However, if the car owner wants the job done now, they will need to pay going rate. It's like that in dealing with any business.

Salvage yards check out engines then usually break the car up so don't expect a drive around the block.

You make it sound like every shop is out to get you. If you quit your job today, bought 10's of thousands of $ worth of tools, bought a building and everything that goes along with running a business. . . . what would you charge for the motor swap and what sort of warranty would you offer?