Boycotting can actually hurt the union, if market conditions suddenly worsen, it doesn’t matter if it’s because of boycotts or market conditions, it takes away some of their leverage. So I wouldn’t stress it too badly.
Organizing your own workplace and participating in solidarity events if you’re able are good things to do to help. eBay’s union has a Twitter, @tcgunioncwa I think it is, they sometimes post calls to action and such that are a good way to directly support the workers.
not too knowledgeable on the subject but i had a similarity in mind: in chemo, sometimes to kill the bad cells, u have to hurt the good cells too, and after an afterthought, u might have to fully kill the tumor so it doesn't metastize and spread. Ebay should be no more if its business model depends on busting unions