The IIfx is a really cool machine, and I'd like to get one myself, but it can be a difficult old mac to own.  The ram is unique to the IIfx, it's unlike the ram for any other mac or pc and can be hard and expensive to get.  This may not be an issue if the one you are buying has lots of ram.  The fx also requires its own special scsi terminator, which can make hooking up external scsi devices a chore.

Is it worth $320?  That depends on you.  It seems a little high to me, but I haven't really been looking.  It is fairly rare and quite desirable, so that price may be in line with what they're going for.