Really there is no niche for ex-librarians when people don't read books anymore. That is why I don't own say, a bookshop. I've worked in one! It closed down, and didn't even last 5 years. The rents are high and you need to do it 24/7 it seems, though this was a chain/franchise independently owned one but I do see how much work it is.
The second hand ones don't make much profit, and op shops sell second hand books too, they get donations and sell them for $2.
Plus online has put the death knell to bricks and mortar, not to mention the behemoth that is Amazon. And even if you owned something like Amazon, you'd need a huge loan, to buy a warehouse so you can store all your books, and teams of people picking and packing, and teams of people doing all the admin, cataloguing and ordering. That is a lot of work, and a huge responsiblity to be an employer. And then you have to work out all the taxes you need to pay etc.