Is it legal to do accounting in the U.S. for foreign companies?
Ask about the nature of their businesses. Speak to the customer (and check each one out, too). The last thing you want is to find yourself unknowingly in the middle of a money-laundering scheme.
You may also want to speak to your bank about the size (and regularity) of the payments you expect to be making to the foreign companies--U.S. banking regulations may have stringent limits on the amounts that can be transferred back and forth.