If I were going to automate something like this, I'd have a suite of products to watch for - common enough to be reasonably frequent but obscure enough to be mispriced, kinda the whole idea behind secondhand ocmmission / antique / estate sale shops.
I don't know how EBay is supposed to differentiate automation from real users in this scenario. To get around it, all you need is human intervention at the last act, so you could fire up your bot and have it forward the "buy now" link when all parameters are met? Maybe they just don't want AI companies to have an argument for some sort of revenue sharing or commissions.