Sadly I can't afford it, and I already have a similar rifle anyway, but just to clarify, I don't believe it's possible that the problem could be anything other than the mag spring being weak from being cycled. If it was anything else, it seems like the malfunctions would be happening when the mag was more full too. FAL mags and indeed other parts are cheap, so it's not exactly a big deal.
As I recall, Century FAL's use either Imbel recievers, or their own production recievers, and the latter has a lousy reputation. If it works right it works right, but it will definitely be something a prospective buyer will want to know. Also, the type of scope mount and scope would be good to clarify. And obviously pictures would be nice if possible.