Because only motivated sellers are willing to accept an offer below market value.

Has anyone used Shopify to build their own STR property website?

SEO is a commitment did your not dedicated to that type of marketing use a basic website.  

I would say no - that a PO box is not acceptable for rental history.

A great blog catches their attention, encourages them to visit your website, and introduces them to your services.If you’re running out of ideas, here are a few blog topics that could work well for South Ontario Windows & Doors:"5 Signs It’s Time to Replace Your Windows" – Help customers identify when to upgrade."

So accepting and approving multiple applicants is the norm?

Is there a website with just seller-financed listings that I'm not aware of? 

However, Montgomery County as of yet has not updated their forms or website, included the landlord/tenant handbook and they are still not enforcing.

Tell the tenant you will only accept payment in full with certified funds.3.

I don't think this is something new, for the past few years I noticed the first 48 hours I post a rental listing I get a few emails asking if I accept section 8.