Before I ask the tenants to pay, I usually go through their application quickly to scan for red flags.

Needless to say, the city posted a red tag notice on the door 10 days later and the tenant could no longer enter the home.I can honestly say that the home was safe and I would have no problem living there personally but the city is furious that permits weren’t pulled and this home seems to have had quite a history with the city (not in a good way).

@Michael Pitsos It doesn't appear that you're considering other expenses that will drive your negative cash flow further into the red such as capex, repairs, vacancies, management, etc.

;age of the building;have there been any updates to the building envelope over the years (insulation, vapour retarder/barrier, fenestration, etc);are the radiators the older, freestanding cast-iron type, the mid-century baseboards, or the more modern flat(ish) panels;are the hydronic heating lines copper, black iron or {red} brass?

The contractor even agreed to take credit cards, which meant I would earn cash back, and I could build business credit with my Amex charge card.Then the first red flag came with the contractor.

Does the sheriff sale signify a red flag or something?

Anti-social personalities (aka sociopaths) can fool the best of us but I ignored the red flags, so this is on me. 

I'm seeing red flags here.

The more of the following boxes they check, the more of a red flag it is for me and the less likely I am to accept their request:  - No message with the colleague request- They're from out of my local area (and usually not even the same state)- They appear to have a sales-type job (an indication they may just want to solicit me for something)- They have zero posts in the forums

Another red flag for a lender.Perhaps you could get a Hard Money loan from some of the commercial platforms out there.