An editor from Merriam-Webster
tweeted that
mercy and
mercenary are related—they both come from the Latin
merces, 'fee, wages, price paid'.
That was new to me, so I decided to look up the root. The nominative plural form is
mercedes, giving us the personal name and car brand name.
Merces comes from
merx, 'merchandise, goods', which is (obviously) also the source of
merchandise. A
market is a place where merchandise is sold, and
mart is simply the Dutch form of the word.
The name Mercury possible also comes from
merx, though it could be an Etruscan borrowing that was influenced by this root.