Good idea. I seem to remember that when I needed rings for a TX200 / TX200SR I bought 'in bulk' .. maybe 50? then sold them on here or at the local club at cost. I did have to charge postage though which put the price up a bit but still cheaper than buying elsewhere when you consider that a 'airgun spares supplier' of one or two rings would also charge postage.
I guess that a drawback is finding enough people who want what you have. For the rings I mention, I never did sell enough rings to recover costs but since then I have used them elsewhere and given some away.
Cheers, Phil