One of my oldest affiliate relationships, and by far my most reliable, is with a merchant that does this one simple thing:

If the product is no longer in stock, the link automatically redirects to similar products, almost seamlessly.

Much better than getting an "Out of Stock" message or something even uglier.

Yeah, I know, keep my products current. And I do keep products current via cron tabs on database sites, which is for the most part the vast majority of my links for the last few years. But for sites not on such a database, it is very difficult to keep all links current all the time.