You can certainly post them, if it is by air you should totally empty the gun (you probably should anyway). I have had a couple posted to me, the last one was specifically sent not by air, surface mail only, even tho it was a springer and i think that it was royal mail who said that it could only be surface mail. I dont think you are liable for the laws of the receiving country, but the recipient certainly will be, i dont know about holland, but to france (you can actually send them to france without problem if the recipient registers them later) and sweeden there is a 10 j power limit, and if they coustoms are doing their job correctly they wont allow an air rifle over the local limit to be delivered to a person without the correct documentation if it exceeds the local limits. so you should ask they buyer if there is anything relevent, or he wont get the gun and will probaly get a summons.
tim