Shop has a MP40 used, what should I check for wear before handing over the credit card?

Bluing wear on the barrel hood, rust on control and slide. Mags should be a matte black with a light grey follower, shiny mags were prone to rusting easily.

Take it down if allowed (and know how, you can take them apart without the tool, just lock it back, hold down takedown lever, release slide forward and pull the trigger and pull off slide.) look inside the frame for any visible cracks, examine barrel lockup for use.

The M&P is a pretty strong and durable design and you really should not have any issues with a used one. I have 3 M&Ps, 2 FS and a compact. My first FS is a Julie Golob edition that has 1900 rds through it, with only 6 cleanings (firing my dirty cast reloads) my new FS, which has the APEX Tactical CompAEK in it has had 1250 rds since July, and my compact has 750 rds thru it and all three get rarely cleaned and look 90% at least

Very thorough answer, thank you!