Posting air rifles 12ft-lb in power and below, is both legal (at present) and straight forward. THOUSANDS of such BBS transactions take place each month without problems. I have personally been involved in over fifty. The way it works is this. First you fill in your profile so people can get an idea of who, where and what you are (age etc as this is important given the new laws).
Then put your price on the advert stating either "including P&P" or "plus P&P", wait until a keen buyer has made an offer you both agree on, request payment by post, and if cash sent by Special Delivery arrives, package and send the sold item the same day. If a cheque however, allow seven days to clear and then send the goods once your bank has confirmed the funds have gone through.
Virtually all rifles are within the max length couriers will accept, so no need to strip. If you do not have a suitable gun box to post in, ask at your local gunsmiths/shop. Usually they will give you one, or -- as with me once -- ask for a token £2 for a suitable box. For the average weight rifle combo, Royal Mail charge £19 Special Delivery. This is a secure system and insured. Parcel Force also offer a 48 hr secure service for £13.85 if you or your buyer want a cheaper option, although Parcel Force only insure up to £150 I think ( check on this first ).
After a "near miss" situation in January, if I am suspicious, I also ask anyone I am selling an air gun to for an e-mail affidavit to my e-addy in the form of a confirmation statement that they are over seventeen ( it is illegal to sell to anyone below this age and you
will go to jail if you break this law
).