There are a number of auction sites that specialise in model railway stuff - example below (use the Filter to select N Gauge lots). Estimate prices are often inaccurate and there's a 10-20% buyer's premium to add on top. In N Gauge it's often older stuff that's offered. Nonetheless there are bargains to be had and it's straightforward to bid online.
https://www.vectis.co.uk/model-train-sale_66 (https://www.vectis.co.uk/model-train-sale_66)
This aggregator website is used by most auction houses and you can set up alerts for model railway stuff.
https://www.the-saleroom.com/en-gb (https://www.the-saleroom.com/en-gb)
These auctions are frequented by traders and I have seen quite a few items reappear quickly on eBay at a hefty mark-up!