While I don't forbid selling invites, be very careful who you sell them to. I may ask you for compensation if you invite someone who is a fraud. Otherwise, you could be banned along with the scammer and anyone else you invite.
You could charge a fee for invites and "insure" new members by holding these fees as collateral for any problems. But, I doubt .01 BTC is enough.
Much like certain private trackers - if the person you invite doesn't maintain a certain ratio, you yourself gets penalized and possibly banned. I think that is very fair and should deter most sales of invites if the person themselves are legit...