Ok - I have bought and sold through Holts. They are professional and will do a good job but are expensive. The commission that you have quoted is excluding VAT (sales tax) which you ought to insist is not added to your invoice: at 20%, it bumps that 25% commission up to 30%. You are not liable to pay as you are a foreigner to the UK. (Although you will be liable to pay it on import to the US.)
There are effectively two UK shippers of rifles:
HGSS (ask for Harry Gordon) and
PDQ (ask for Bob Nagra, although I have heard that he has retired). It might be worth asking the likes of Rigby or Westley Richards if you can tag along your rifle with their own consignments to the US shows in January, on the basis that the worst they can do is tell you no in the form of a two-word expression.
The UK export is not a problem. The customs are, in my experience, surprisingly helpful as long as the paperwork is in order (and Holts will see to that).
In answer to your questions:
1 - That
ought to be sufficient. I have always found Holts to be helpful and they will advise. If you go through one of the recommended shippers, they will/ ought to deal with the issue on your behalf.
2 - import duty and sales tax. I am not an expert on US taxation! Note that import duty (at least to the UK) will be calculated
inclusive of Holt's commission and the shipping costs.
3 - Yes
4 - It is a long time since I imported from the US, and I have never sent a rifle the other way. By way of comparison, the shipping cost of a rifle from the UK to South Africa is roughly £800 - 1,000/ $900 - 1,200 at the moment, I believe.
I would make two other points. If the rifle is for yourself, and you are not looking to sell it on, you have to work out (1) how much you want it; and (2) how rare it is. If you really want what is for sale, close your eyes, grit your teeth, and accept that you may have to pay over the odds.
BUT: the great thing about actions is that they are like busses: you miss one this time, but there's another one coming along later. So if what you are after is not rare, you can accept that you will miss it this time, and something similar will come along later.
Second point: MOST IMPORTANT: get a condition report before you go mad at the auction!
Good luck, and Happy Christmas. Let us all know how it turns out!