Lol, your googlefu is stronger than mine.
General rule of thumb for s/h stuff in airsoft is considered to be 60% to 70%ish of the cheapest new price at the time of posting.
But depending on supply, demand, or even who's using the kit in active service at the time can cause the accepted percentage to fluctuate.
Problem you have is irrespective of rrp, not many airsofters splurge £200+ on load bearing equipment of any sort, not with so many good quality copies around for a fraction of real steel prices, with the exception of the hard-core stitch counters, but they're all bonkers lol?.
Add to that, you've stated yours is a "desert camo", but which one & who uses it ?, that kinda info may give you a better idea of a target market, whether it's airsofters, reenactors, or even the home market of whatever country or units use that camo ?.
Good luck, it may not be easy to shift but I sold an Israeli vest to someone in the Lebanon once, at a considerable profit, so anything is possible ?