Make sure their country has a tax treaty with the IRS, most countries have this. Only scam countries like St Kitts don't do this (I've ran into this) . If their country doesn't have a tax treaty with the US you may be liable for 30% witholding of their taxes. At minimal you should make them do a W8-BEN. Also non-us entities can get us tax ids (TIN), they suggest this, but not sure how much this actually helps.