You generally want to aim for 60-70% of new price for a used airsoft gun in decent condition,
if you can find one. Sellers will always ask more and claim that their shagged out bag of loose bolts is "basically new" or "just test fired once", while buyers are always looking for a bargain.
If you're after a specific model, you can always stick an ad in the wanted section here -
https://airsoft-forums.uk/classifieds/category/17-guns-wanted/ - but you'll need to be very careful about scammers claiming to have just what you want, when all they have is a stolen stock image.
Buying used is always a risk, and it's something I'd only encourage if you're willing to work on the gun yourself to remedy whatever it is that's prompted the owner to sell it. Buying in person and testing it first is a better idea, and it's worth asking around at your local site(s) to see what people have for sale.
Clean the barrel, dial the hop in, and feed it the heaviest BBs that it will hop and that your wallet can stand (I tend to use 0.28g, other people go heavier), and you'll be competitive with anything else out there.