I supose it all depends on why the person is getting rid,if its someone who has worked them regular, and is, say packing up then why not,
Regarding adult ferts, i was out a mooch years ago, walking along a flood banking, when i noticed rabbits bolting, i hung around thinking it was a stoat, when out popped a Polecat Jill fert, i picked her up, she was tame, and turned out to be a cracker.
I also bought the odd kit at Lowther, and never had one that didnt work,
Interesting Tote mentioning Lanark market,this market has always had a good reputation for picking up good Ferts.