So, should I assume that the reputations of this whole forum, as well as of it's posters, are questionable reputations, just because there are advertisements in each and every page?
Absolutely.
And as a user of the forum, you should pressure the owner to remove those advertisements that you feel are for questionable services.
If the owner of the forum refuses to remove advertisements that you find to be of questionable nature, you should refuse to participate, and should either find another forum or start your own.
I would disagree. Advertising is something necessary to pay for expenses of content creators.
If the forum itself is not able to advertise then it would need to resort to charging users for accessing and contributing to the forum which I am sure would not go over well.
Also the term "questionable" is a subjective term. Any person could consider anything to be "questionable" if enough money was involved. I do see that you are advertising your escrow thread on your signature. While you do have green trust and would trust you with several thousand dollars to be held in escrow, I would likely not trust you with 5 or 6 figure (measured in dollars) amounts in escrow (I would likely instead opt to do any deal with you as escrow in smaller amounts). Since I do not see any dollar (or bitcoin) limits as to how much you are willing to hold in escrow, I would say that, in certain circumstances your advertisement may be considered to be "questionable" (there is at least one auction for 200+ THs worth of miners being sold for $100k+ worth of bitcoin (I couldn't find it in either the auctions or computer hardware forums, but I know it is somewhere - there are a lot of domains junking up the forums).
I hope you understand I am not putting your integrity into question, and do not think that you would likely steak $10k+ of bitcoin if you had the chance