I am the buyer.

Marketable title is title that doesn't have material defects... and as such the buyer cannot claim there is a title problem and refuse to close. 

I'm not sure why they can't at least give you some pics showing the condition and layout of the inside of the unit.When I market my properties, in my ads I show pics of the inside only (no street or address, just city neighborhood), but once I start a dialogue via email with a prospective tenant I give them the address so they can do I drive by. 

This Contract Is Not assignable by Buyer unless otherwise specified inAdditional Provisions.So in the additional provisions area you just state that the contract is assignable (with or without perimeters). https://drive.google.com/drive/folders/0B1VD36mBqe1EfnJaMGl3MVBxX0Zva3RVbEY2amVuc1Y4SFh2X3JDZTVHeGxUUVRzaGdDMVU

I have had one realtor bring me buyers for five different houses in the past.

In most states, including TX, the owner must disclose the inspection to future buyers, by law, so he/she loses and the future buyer gets the benefit of my hard earned money. 

He might be right, and since we are first time buyers, it makes sense.

I have found a buyer that is offering 36k.

You were essentially not in business unless you were a true cash buyer though.  

There are several approaches to doing lease options, and consequently, many variables... so it would be prudent to do so to 'maximize' the exposure, pre- screen the largest number of applicants, increase the likelihood of 'closure' (selling the property) with the tenant buyer(s) and structuring the deal, to optimize your ROI.