I assure you plenty of people and posts are blocked.

I can assure you the new landlords are judging you when they view the property.As for the landlord you need to communicate it to them. 

I have a potential buyer who (I do not who is he), although he is interested to buy the property, and he just send me the Purchase agreement for me to sign it.In order to prevent a scam what are the actions and/or information I need to ask to assure a smooth transaction?

In the reality of the market, it is very difficult to find deals that will assure that you get back 100% of your initial investment.

Like I couldn't tell you how many posts or votes @Chris Seveney has, but I assure you if you want to know anything about notes he's the first guy I'd refer you to.

I'm conflicted because it was non-violent (shades of gray at worst), 5 years ago already, and they're assuring me his record is clean since (I have not run their background check yet since I wasn't going to make them pay for that unless I thought I'd actually rent to them).

@Marco EgurvideHaha, I can assure you they're legit!

I guess what I am trying to say it is not totally unusual to have a problem tenant in your building when you first purchase it.I know if feels difficult right now, but I can assure you - it well get better.  

Just be as sure as you can be with your numbers.

Assuming you have the funds actually in the account.Also offer to show proof of funds in the account to give the seller total assurance you are a safe choice.I encountered the same situation before.