> should they be disclosed?

Yes.

> Isn't that assumed nowadays that every link to a marketplace is an affiliate link?

Other people doing something wrong is seldom a good reason to do it wrong yourself.

My own personal pettiness: If an article declares the existence of affiliate links, I'll use those links more often than not. If they don't, I'll make an effort to revisit the links without the affiliate IDs. If an article presents both affiliate and non-affiliated links, I will generally use the former, and I'll trust the writers opinions a little more than otherwise. I actually keep a separate browser for buying things once they have been researched, to slightly inconvenience the tracking of me generally, so I won't be linked by “last affiliate” tracking unless fairly decent profiling is in action (which it won't be: sellers won't make that much effort just to pay money out to affiliates), only if I copy over the affiliate-id decorated link (or the original source article and click the link in that environment).

Social enforcement is probably all we can get, but yes, they should be disclosed. I'ma a fan of using 'affiliate link' as the anchor text, but it might offend one's html sensibilities. A brief one sentance blurb about commisions before the affiliate links start is sufficient.