The moulds for resin models have a shorter life apparently relative to the moulds used for the cheaper "toy-like" plastic models. This gives the more popular resin models an intrinsically higher value.
Still, if something is selling well, I can't see the logic in not creating a new mould for another batch. The original manufacturer doesn't stand to make anything from high priced sales on eBay.
Came across the following interesting shed in my search: http://www.ehattons.com/60728/Bachmann_Branchline_44_173_Brick_Servicing_Depot_320_x_82_x_121mm_/StockDetail.aspx
It reminded me of a similar shed from Lima many years ago., a few of which have popped up on eBay recently.