Easy for me to say this with hindsight but it sounds as if you bought with a 90 day guarantee and waited until almost the end of the period before trying it and now the 90 day period has expired.

Stating the obvious but with second hand stuff like cameras always run a film through it within a day or so of getting it so any issue surfaces straight away.

If the seller is decent he will give you the benefit of the doubt but from his point of view he may wonder why it wasn't used before now or be suspicious that it was used, was OK and something has only just gone wrong.

I hope the seller will "play ball"