Hmm thats a good point. But I am sure you could still implement BIN in the forum whilst still not allowing itrader to be left for the threads. It just doesn't make sense not being able to choose a BIN option in a cheap stuff forum when in almost every case it's a fixed price.
Yeah, it would make more sense to have instant pay available also. Surely it would be easy to code a check for exclusion? I wonder if it's part of Shawn's plans for when iTrader is finally removed?
My guess is that's the point. No BIN is a way of making it unavailable and therefore not possible to record a purchase (under instant pay), rather than just ignoring it, although it doesn't make sense to do it that way. Hopefully Shawn or a mod can clarify.
The real problem has to do with how originally the auction system was just for sites/domains, and instant pay wasn't part of it at all. So in merging together all the unrelated stuff (into categories they really weren't designed for), there are some internal issues/stupidity that crops up like you are seeing. The whole system is being redesigned/rewritten from the ground up to "mesh" better.