Turnover is one of the biggest, you generally need to do some renovation and advertise and show it, but you cannot do them at the same time. 

or can you put a property under contract but not advertise it (since you don't own it) then pass the contract to a buyer.?

Monthly expenses are at a minimum, insurances (liability at the very least), taxes, business fees, health department fees, road and infrastructure maintenance (hold back for one-time expenses), management, utilities, advertising, etc.  

HAPCO has some really good documents that spell out the Philadelphia regulations that a landlord must follow.And if you are getting a PM, the HAPCO newsletter has some PM advertisers - but there is one PM advertising in the HAPCO newsletter that you would best avoid; I can tell you that privately if you need to know.

If you are advertising there you may want to double check your board rules just to be safe.

How would you advertise these rentals?

The guy advertises as a business, and I paid by check both times, no complaints from him. 

 There is nothing more annoying in the advertising world than re-targeting.Oh look... there is that thing again that I decided Not to buy/do with that vendor I didn't want to use.

bad advertisements?

Owner did a horrible job of advertising and it's been for sale for 4 or 5 month now.