My late father collected stamps and was something of an expert in the areas in which he chose to specialise. (e.g. he could have many different versions of what seemed to the same stamp in his collection, but with each one having slight differences in the colour, perforations, phosphor bands, plate numbers, etc,all of which were carefully annotated alongside each stamp).
When he died I took his collection into Stanley Gibbons in London, to see if they were either interested in purchasing it outright or selling through one of their auctions. They simply didn't want to know. (I ended up selling it all to another dealer, just across the road from Stanley Gibbons, for a tiny fraction of its value, simply because I'd lugged lots of heavy albums to SG's on a boiling hot day and couldn't face the thought of dragging them around London with me for the rest of the afternoon).
If I was seeking to sell stamps now, either singly or as a collection, I'd use Delcampe. It's the world's leading specialist site for such items. (I've used it for postcards):
https://www.delcampe.net/en_GB/collectables/