I've never had too much trouble with fake games bought here in Ireland. I've gone on holidays to Spain once or twice though and been all "OH COOL A POKEMON GAME I'VE NEVER HEARD OF BEFORE!" and have bought one, I have no idea what it was called, but it was mostly in Spanish and didn't save. I wasn't too bothered cause it only cost €15 or so, so no big loss.
€100 is WAAAAAAY too much for a second hand Pokemon Game. Heck, I wouldn't pay that for a legitimate day-one-packaging, brand new sealed, never been used Ruby version.
On a side-note, Pokemon Ruby secondhand, cart. only is €12-18 here in Ireland. Surely the battery isn't that hard to replace even if it did run dry?