Each can have their own STR regulations.San Diego city (the city council) last year passed onerous STR regulations that basically banned STRs where the owned did not reside on the property. 

https://www.google.com/amp/s/www.telegraph.co.uk/news/2017/01/06/britains-biggest-landlord-has-banned-battered-wives-plumbers/amp/

b) An 8 cap purchase that is well-run, all fixed up, has great amenities but is located in a market with declining rents and the agency lenders are not funding new originations in that market. 

Would a private money lender b an option?

@Maxwell Fontaine is absolutely correct, many HOA's will restrict or ban renters 

These cities are much more likely to enforce bans or heavy restrictions on STR in the near future.

Airbnb is extremely time intensive, but you can net about 2.5x what a traditional rental would. most HOAs ban STRPs whether you own it or rent it. 

The measure says they will ban vacation rentals in most of the city.

HOAs can ban rentals altogether, have a rental cap (ie max % of non-owner occupied units), or no rental cap at all.

(a) (1) This chapter does not apply to:(A) An owner of real estate with respect to property owned or leased by such person;(B) An attorney-in-fact under a duly executed and recorded power of attorney from the owner or lessor;(C) The services rendered by an attorney at law in the performance of duties as an attorney at law;...