During battery replacement for my Suunto computers, I don't always bother to replace the o-ring.
On my Favor computers, I've never changed the o-ring (2 sets of battery changes over 10 years).
On my Mosquito, I'll change the o-ring with every other battery replacement. I purchase the official Suunto kit, which includes the plastic battery hatch (easily damaged during battery replacement), new o-ring, and replacement battery.
On my Vyper, I've never changed the o-ring (no battery changes yet, purchased used 2 years ago).
On my girlfriend's Gekko, I've never changed the o-ring (1 battery change over 6 years).
I would think that if the Ebay kit includes an o-ring, it would be the correct size. When in doubt, you could certainly use digital calipers to measure the o-ring size.
I've heard Vytec owners state that the battery o-ring is Suunto/Aqualung part #K5664, 1.78mm X 31.47mm. The o-ring is ShoreA 70 durometer.