To summarise looks like you’re down £150 and lesson well and truly learnt
EDIT: Having said that, I’m a regular on STW, but on the odd occasion when I’ve sold something of higher value ie a frame, I’ve posted it on pinkbike and bikeradar too, so to them I’d look like a dodgy classifieds only possible scam type character. Just maybe there’s an outside chance that there’s a genuine mistake…?!