It depends on your country and on how the seller sends the coin. Usually sellers will send with registered shipping and a declaration of the contents, which makes it likely that it will go through customs, again depending on your country. Then it might either be processed automatically. This usually does not mean anyone touches the actual coins. Or you may have to go somewhere to do the process yourself, usually if the necessary paperwork was not in order. In this case, you yourself open the envelope or package and the officials will not touch your coins.
It's usually a 7%
VAT for collectors' coins but sometimes there may be trouble. Here in
Germany they occasionally select the wrong entry in their classification system (like "
antiquities, more than 100 years old", which is not wrong but there are better matches). In that case you have to argue, which usually works but is exhausting, or you have to pay the higher
VAT. This can really hurt.
If it's a small, private seller that you know, they may send your coin unregistered, in a normal envelope. But I would only do it with people I know, because if the buyer claims they have not received the coin and they paid with Paypal, they can get their
money back from you. It's happened to me several times.
Currently I have a shipment of coins that was incorrectly classified and now I'm trying to get my
money back, almost 100€.