Perhaps there is one, but until that can be provided as justification for their decision it doesn't seem proper for USPS to be playing 'detective' telling people they can't hang a mailbox when there is a physical address on record with the county.Has anyone run into this too?

It is not worth the time or effort IMO to be a private detective. 

I understand how important it is to expand your presence in the online marketplace to reach a wider audience.Given the complexity and technical aspects of e-commerce, I would advise you to seriously consider enlisting the help of a professional e-commerce development company.

But I would not let him out due to a smell only he can detect.

I'd go in person and talk to a detective and see what they recommend you do.

I think it's the best way to get started when you don't have a ton of money to spare, and you are still learning.1. look up on facebook marketplace what other people are offering their rooms for, that will help you get an idea of what you can charge. 

Tried Craigslist and FB marketplace so far.

I told this to one of my friends and he gave me the advice of looking up properties for rent on FB marketplace and Craigslist and calling those owners to see if they would be interested in selling their property.

It's not your job to play detective,  or the police, just let them know you can no longer offer them a place. 

So it pays to learn the marketplace well.