Yes. It's legal to shop abroad. Lots of exchanges do exclude Americans but if that is the case they will state you're not welcome on sign up. Bitfinex, Bitmex and no doubt a few others won't welcome US residents. I'd imagine the smaller exchanges like Cryptopia don't care yet. If they do you'll know.
Yes, Cryptopia does not mind at all where you are coming from as long as you can do business in the site then you can be a member of it...they need more our loyal patronage more than anything else. of course, this might be temporary as countries right now are really demanding more transparency from exchanges. USA is leading the way for implementing KYC requirements and banning other people from joining. Anyway, there are many choices right now sprouting so we might as well go shopping and find which exchanges are we comfortable with and sure that will not run away at a slight problem.
