I think you are pretty right on there, but SLIGHTLY shirt-changing yourself. Lets boil it all down...
Right now a LIKE NEW Glock 27 could probably sell for $600. Ive seen new Glock 19 gen 4's going for $800 but thats because 9mm is the #1 most popular semi cartridge. This is a .40 cal pistol, slightly less desirable, but not much. However, this is a trade proposal so always start high and work down...
So Lets say $650 for the G26 is what its worth on a gun auction.
- $300 cash
- 200rds of even cheap .223 ammo right now is selling on average for $0.85 per round. So thats about $160 in ammo
- 30rd PMAGS are selling on average for $50/ea so thats another $100
You are about $100 short on this deal, working against you. I would ask for AT LEAST another $50 in cash, or another 100rds of ammo. Just my $.02. Remember, start high and work down