Just to follow up with Jeff's footnote. I own 5 Zenith's total: 1 Pocket watch (that required a new mainspring
shortly after purchase [but the watch dates from pre-WWII),

an HP146 (which is a movement comparable to the Valjoux 72, Lemania 2310 (Omega c.321), and three El-Primero's.

Of the three El-Primero's the only servicing any of them have needed was my second one (the Pilot-Diver's) required
the under crystal tachy bezel re-zeroed. I bought it from a local seller that way, and the local Zenith repair center
charged me $20 and fixed it on the spot while I waited. They even examined the movement said it was in great shape
the oil looked good and pressure tested the watch.

They all have worked flawless for me otherwise.

Frankly, purchasing used is a bit of a crap shoot, I've been fortunate I guess.

Cheers and good hunting!

-- Chuck