I have imported many air rifles and pistols from all over (eastern)Europe/Asia and US to the Netherlands. It is indeed the importers business to consider legal issues.
Finding a carrier is not too difficult since there are many available. I have found there is no pressing need to reveal the package contents, unless numbers and stakes are high. If the parcel is scanned or damaged during transit,
it usually takes a bit longer and damage is not covered. In all the years, I have always received the package, but sometimes damaged or the item was in an unexpected state.

Simple things that you can do to prevent some sort of conflict (probably obvious for most of you):
- Damage may occur during shipment. Agree on who is taking that risk and how/if it is covered. if you agree on returning an item, who is paying the fees. I normally have the stock and action separated when an air rifle is shipped to me. The package is more compact, better to protect against damage (leverage for breaking the stock)
- Make sure both parties are aware the state of the object. High res photo's help, but discuss weak points in advance.
- International payment: both parties are served by lower money transfer fees. Usually fees are involved for either seller or buyer: Paypal friends & international transfer in foreign currency ar at buyers/importers cost. Fees are limited though (5 -10 pounds). I have found that transfer in Euro's to a foreign account is cheapest, but you need to calculate the correct amount (buyer rate Euro's of recipient's bank). Bank transfer needs an International Bank Account Number (IBAN) and sometime a BIC code. All European account numbers were transferred into IBAN numbers a while back. It is on any of your bank statements or it can be derived online.
Some people are just not comfortable sending an item abroad, so I also have a friends address available locally (in the UK, US and Germany) to make things easier.
- Knowing the opposite party a bit better always helps. Part of the people I dealt with I got to know a bit better through special airgun interest-groups or by knowing fellow members. The Facebook Vintage airgun groups are a great addition (do join!).

Hope this adds to the discussion.