Notably suffix -er, -ere, -ero, etc. (such as in carpenter) It does not seem to have a PIE origin but there is a very plausible Vasconic root: -ari.
-ari does not just have the same meaning as -er (futbolari = football player, aizkolari = ax worker, nekazari = farmer, etc.) but unlike -er it does have a plausible Basque etymology from ari(-tu), to hurry, also used in verbal forms of activity: (s)he's doing something: zerbait egiten ari da.
The etymological sequence should be as follows:
aritu (to hurry) <-> arin (quick, fast) <-> ari (auxiliar verb of action) <-> Basque -ari (professional suffix) → proto-Germanic *-arjoz and/or Latin -arium → modern -er, -ero, etc.