From my experience, it seems like cash is the 
best option.  Most of the big dealers with accept 
both credit card and cash, but if you pay with 
cash, sometimes you don't have to pay tax ;)

As for trading, most people carry around cel 
books.  The collectors who have a LOT of cels 
sometimes wheel around a suitcase with their 
books.  If you have a lot of high value cels that 
you don't want to travel with, i suggest making 
good color copies of them and maybe try to trade 
that way.