interloper

One who interferes, intrudes or gets involved where not welcome, particularly a self-interested intruder.

They disliked the interloper, and forced him to leave.

In a button-up plaid shirt with an unwieldly suitcase, he is an interloper from another world – one whose first social faux pas is to make his way slowly through the various gates and arrive at the imposing front door on foot, rather than wait to be picked up from the train station by a servant.

An unlicensed or illegitimate trader.