Yeah, take it to the range and fire off a load of bb's before you crono, my guns rarely get used so the springs are pretty tight for at least a hicap mag worth, for GBBR's you can fit an Npas so you can adjust the gas flowing forward or back.
I've bought several items valued between £5 -£300 from the classifieds here & the only issue I had was on a low vaue item (about £10-20), the seller refunded me with no issues & I offered to pay again if it eventually showed up.
You pay your money & take your chances.
TBH I've had more issues with deliveries from retailers.