I'm not sure how eBay reinserted OBO. What was their reason in their email? I haven't noticed that before on any of mine.

As for shipping, I usually build the cost into my price. If you have bulk lots of stuff, does it help to sell an item in pairs or 3x to combine the shipping cost to squeeze out more profit (as long as a quantity discount doesn't hinder that extra benefit)? I saw your dialogue with @getonthetrain below who pointed out some of the other points I would have made.

I suggest listing some items on Craigslist and Facebook Marketplace for cash as an experiment.

Since you mentioned wanting to try Mercari, has that shown any promise if you started there?