Welcome
Do lots of reading on the forum. Search for various topics and you're likely to find answers.
Research is the key to getting a good deal, not wasting money and not being disappointed...
Could help to get in there and clean it up as well as re-shim.
O-ring nozzle is good as long as it fits properly (length and not too tight)
You could also try polishing the inner barrel.
(Presume you already cleaned the barrel...)
Got my son an RMR but the only thing I got was a pair of socks I can wear at the next game ?
Hopefully I can complete my order at Taiwangun next year ?
You could try reporting him to ebay first. That might delay things a bit at least.
Next time specify 'Tested working', 'No returns' in case that helps.