I totally get what you are saying. Of late I have bought a number of items from OS, very cheap and $0 postage.
Good for me a s a buyer but not good for local sellers. Especially now buyers are getting used to having long delivery times.

Unfortunately I think the biggest issue all round is that buyers simply don't have as much money to spare as they once did and would rather wait a few weeks for an item rather than spend $5 on postage.
Also some local sellers haven't helped the situation by flooding the site with items they themselves have bought  cheap from OS and now selling for more plus postage. Once upon a time buying from a local seller meant you would get a better quality item than from OS.
Sometimes I think the old days of the OS postage pirates at least kept it a level playing field if buyers were smart enough to see what was going on. So whilst postage pirates were bad for buyers, they were in some ways good for the honest sellers.