No, Royal Mail policy has been changed regarding prohibited items, and they are enforcing it.
Courier services will allow - but check their policy.
As a buyer its not your problem, the seller is responsible for getting the item to you.
Pay through a protected method (e.g. PayPal), don't avoid the fees, have the seller use an appropriate method for delivery with tracking.
If it all goes wrong then you reclaim back from the seller via PayPal, they get their compensation from the courier if it was lost, they lose out if they ignored policy or mis-declared.
Alternately buy new from a paintballer retailer, or at a stall at an event.
A 3000psi aluminium / steel cylinder is finacialy a best buy at around £30-£40
Second hand 4500psi fibre cylinders average out at around £80 irrespective of its depriciated / lifetime value.
E.g. a new fibre cylinder is approx £150, it has 2 tests for its 15 year life - £200 lifetime cost.
The maximum value of a fibre cylinder at anytime is:
£200 / 15 * remaining life less remaining tests.
But then deduct the 'secondhand saving'
Postage is an added expense and affects viability. Its no added value for a buyer and is an expense to a seller
A 4 year old cylinder is worth:
£200 / 15 years = £13.33
£13.33 * 11 years = £146.66
£146.66 - £50 for 2 tests = £96.66
A 6 year old cylinder (tested) is worth
£200 / 15 years = £13.33
£13.33 * 9 years = £120
£120 - £25 for 1 test = £95
A 4-6 year old cylinder has depriciated to around the right point for the £80 second hand value