well everyone has already mentioned the important stuff, the rest you will be able to figure out. just be sure to read TB FAQ and rules. you have a part to play as well.
TB sellers are people, so the conditions of their sales are determined by them, which can be anything, as long as they do not violate TB regulations. so we can only advise in general.
if you want someone to ship to a PO box (vpost is a PO box), do let him know, rather than just telling him it's a US (etc) address. it is common that stores and ebay sellers do not ship to PO boxes, so TB sellers may have similar reasons why they too would not.
sellers tend to be reputable, and usually have an impressive history of transactions and feedback. consider the fact that use of the classifieds is not free, you have to buy a supporting membership to post a thread. on the other hand, it doesn't cost the buyer anything.
if you are a newbie, consider that one may not be confident of dealing with another TBer who has no previous history. when the value of the item is very high (eg. a bass), the risk is even higher. this is on top of any reluctance a seller has to ship outside his country.
my first TB deal was trading AMPS with a guy from australia. he was frank in his feedback that he was unsure because i had no previous history on TB.
