The Code enforcement of the City sent a letter to say the tenant needs to vacate and the owner must reimburse for relocation fees in the amount of 2 months rent and the deposit.

Property Managers are just like tenants: you need to know what a good one looks like, screen until you find a good one, understand your contract, monitor them, enforce the contract, etc.

What strategies would you recommend to enforce a stricter garbage policy?

In my brief adventure doing transactional financing, I actually ended up with several loans that were actually usurious and would've been very difficult to enforce the collection of a delinquent note.

So, even if they had good records they wouldn't have a lot of details needed to enforce.

Others include:Rental law enforcement and complianceTenant turnoverInspectionsRaising rentEvictionsInsuranceTaxesBenefits of Property ManagementProperty managers have made a career of understanding the rental industry.

All that happened was that they could not use the court system to enforce their contract after they did not get paid.  

Rarely is the Due on Sale clause enforced.

That's probably not enforced very much but I wouldn't feel great about risking it so I can't just buy a 5 or 6 bedroom house and rent all the rooms. 

I'm a buy-and-rent investor so I want to make sure I have clean title (and can get mortgages) for as long as I own it; TTS might make a lot of sense for a flip.Approaching the owner before the tax sale is worth a try, but a lot of the time the reason it's going to tax sale in the first place is that the property is worth less than the combined liens (tax, mortgage, code enforcement, water & sewer, court judgments, etc.)