Lately, I've been seeing a lot of users posting links and/or advertising their sale threads on other users' threads that isn't even relevant to the topic.

They call it 'shameless' because it's usually starts out with comment that is on topic, and they feel like they can throw their advertisement in their post, and that makes it okay..?
No. It's not. It is off topic and just down right annoying. No one likes ads.

If you're not sure what I'm talking about here's and example:
Say I'm selling artwork and someone posts on my thread with:
"Oh I love your artwork! I may be back to buy some once I have enough GB!
Also, shameless advertisement: "INSERT THEIR SALE LINK"

It's fine if you're linking a thread that is RELEVANT to the topic, but not a random ad in hopes of getting more traffic. That's what the Bump button is for.

My thread, or anyone else's thread (other than your own) is NOT a billboard...

TL;DR: 'Shameless advertisement' should be considered spam and should be treated as such.