Condition means everything here. I would go over the fundamental points of the gun (do a search) such as slide to frame fit, cracks in the rails, etc. See if you can field strip it there. If not then tell him it it might be worthless since Bubba might have done anything to the internals with a file. If you get your way, check. If you don't then tell him you would have been prepared to talk turkey, but a potentially FUBARED gun from a vendor who is acting suspiciously isn't worth more than $325. Either he relents, or he chases you out of the store where you scream "I will not barter my body for a discount you perv!" to all within earshot.
Once you have determined the weapon is satisfactory, offer him $400. If not, tell him you calculated $425 with appropriate sales tax (say $460.31 OTD) is what you brough and it's your bottom line. Flip it on the table as you tell him that. He'll take it.