I guess it depends on the middle man and the amount of work he is willing to do.
There are three options really:
You package for postage to GB then put that into the flatrate box, he opens the flatrate, adds stamps and posts your package.
You send it in the flatrate box, which he puts into a bigger box, adds stamps + customs form + address and posts on.
You send it in the flatrate box, he repackages it, adds stamps + customs form + address and posts on.
You or the middle man should confirm with the buyer re what to put on the customs form.
Normally I suggest it's marked "gift"(check box), "camping tool" and a low $ value to avoid import or other fees.
The catch to low balling is if it goes missing they'll only pay out that amount.
For an idea re import fees etc check out this thread,
Guide for the international trader.
When I got my PM2, I had it repackaged state side and sent as standard mail with a low value on the customs form at my risk.
The plus side to this was saving a lot of money on the postage.
If I was to buy a Chris Reeve, I would have it sent registered with the full value declared and pay the import fees if there were any rather than risk losing it.
Other than making sure you have packaged it well, IMO the rest is on the buyer, they can choose the options depending on the risk they want to take.