Just my two cents:
Archive.org and WinWorldPC — I think it's okay to link to documentation, drivers, BIOS images and software updates (not actual distributions/disk images). Technically it should be okay to link to shareware software as well, but WinWorldPC usually carries registered versions, and Archive.org often mixes genuine shareware CDs with actual warez compilations (not that they don't have huge historical value). It would be unreasonable to expect moderators to check legal status of every piece of software there.
So "here is a link to Windows 95 CD image" — no.
"Here is a link to a fast CPU fix for Windows 95" — yes.
eBay — I'm not sure it's a legal thing, but I might be wrong. Ebay discussions have caused one of the few dramas on this forum in all of 7 years I've been active here, so I think this is why it's a tricky subject. I never fully understood that situation, but I do agree that once you mix retro discussions with a marketplace, usually drama is bound to happen as witnessed by some other forums I've been on.
That being said I think it's okay to link to eBay for purposes other than buying/selling. Case in point — I had a topic where I was searching for drivers and manuals for a certain card. Someone has found that card on eBay and linked it so that I could contact the seller and ask him to dump the drivers. Also, eBay is generally a huge source of photos that can't be found elsewhere. In most cases it's the first thing that comes up in Google Images.
So "Here is where you can buy a ESS soundcard" — no.
"Here is what a ESS soundcard looks like" — yes.
I'm not a lawyer and not a moderator, but I've been following these guidelines and never found issues. I think there was one instance where I posted a link to (I can't remember what it was, but it was either Archive.org or WinWorldPC), then doubted myself and removed the link, and then found moderators removing the reference altogether. Given that I wasn't sure if it was okay, I wasn't surprised to see that kind of action.