Yeah, I'll make the offer next time I'm at the store. Hopefully I'll run into the same employee.
I'm 36. I thought "out the door" meant "great deal," as in, "get out of there before they realized they made a mistake."
Uh-huh. Yeah. Sure. But now you know.
Do not wait until "the next time [you're] at the store". Take the time, burn the gas, and make a special trip there to straighten this out. If you know who it was you dealt with you might call to find out when they will next be there and schedule your trip to coincide with their shift. Ask the employee to bring the manager in on the conversation (as the manager is already aware of the basics, if not the details). A simple "I misunderstood what the term meant" (without saying anything about what you thought it meant) accompanied by an "I'm sorry" and cash pushed over the counter will probably restore you to "good customer" status.
And if they give you some cock and bull story about not being able to take the money because they don't know how to account for it - push it farther across the counter and tell them to use it to go out to lunch on you. Don't take no for an answer.
And I differ on the issue of your obligation to the store. I see it as legal, not ethical. You put the gun up for sale - presumably without a reserve price. That's a contract. (And if it was a good LGS they put it in writing.) If you did have a reserve price the store had no obligation to pass on a lesser offer but doing so would have been good business practice on merchandise that had been sitting for an extended time. An offer was made and as I see it the
buyer paid the sales tax on whatever it came to so that the total he owed the store was $400. If $20 was the sales tax the
buyer already paid - as he should have - the sales price of $380 leaves you with a consignment fee (15% you say) of $57 owed, and you should have walked away with $323 in your hand.
So you may have more than $20 that needs to be pushed across the counter. There is a chance that the counter clerk will be docked the missing money for their part in screwing up the transaction regardless of their attempt to create customer good will without prior approval.
What I'm getting at is that if this is to be a "learning experience" for you, learn the total lesson. In the long run it will be money well spent.
stay safe.