However, there's no harm in interviewing them to see if they might be a good fit for you.

Could be that others you see have similar exemptions which are causing the taxes to be very low, but you can confirm at dcad.org 

At one point I almost walked away from the deal since their delay caused me to miss the deadline for the home buyers tax credit.

I initially figured it was his mismanagement that caused him to lose the property.

Actually there's a lot more to procuring cause.

You can write all the stuff in a contract you want but at some point all you are doing is forcing them to violate it somewhere or forcing them out causing vacancy and a never ending search for the perfect tenant.

One way these people can cause problems for landlords is by calling the building inspectors (see this guy's story for an example). 

It can and does work if you have an honest discussion about things like not having to pay realtor commissions, that they do not need to invest any time or money in repairs, that you can pay cash, that there will be no contingencies that would cause the deal to fall through and in fact you can have closing as soon as they are ready.

In the past, I've found that getting a judgement is sometimes enough cause some tenants find they need to clear judgements off their records in order to accomplish one or more goals.

I was initially leaning towards Las Vegas but I soon learned the investors have drove the prices up quite a bit in these last few years causing my potential cash flow to diminish.