There are very few alternatives. Ebay has managed to maintain a monopoly in this. Gumtree is the only similar site i can think of. I have sold stuff on Gumtree and it is much better and simpler. You just arrange payment and postage with the buyer directly. There are no fees. But the issue is that Gumtree is used much less than Ebay which means that it takes far longer to sell your items. There is a much smaller model railway selection sold on there. But it is worth a try as i have sold railway stuff on Gumtree successfully before.
What I've done on my eBay account is disable Inpost and Evri in the Simple Delivery settings - I'll only ship through Royal Mail so I can always print the label myself for maximum flexibility of collection or dropping off at a post box / parcel box / Post Office.
Since I'm the delivery boy that takes items to the post office, I didn't want to be dealing with the likes of lockers too, so I got my Wife to disable every delivery method that didn't involve the post office.
It doesn't cost the seller (as you have described) so they are perfectly correct. It costs to buy.
People used to state inflated postage costs to make more money from uninformed buyers.