They were sure that their house would close on a certain date post lease-signing, and upon my urging (twice) they confirmed with their real estate agent that their buyer's financing was secure.

We also sold our own home earlier this year, took three weeks to go under contract, then fell through four weeks later (on the buyer's end), sat on the market for another three weeks and we went under contract for 45K less than original asking price.

Be a prepared buyer, not a just a dreamer!

This will help you to be taken seriously and for them to actually add you to their buyers list.

Can the seller sue the buyer for anything?

If you wait for it to go to auction then you will pay more and be competing with other buyers that weren’t willing to put in the work.

I currently have a buyer under contract out in Oak Forest, near Garden Oaks & you can actually find a decent/ single family home in need of minor cosmetic repairs.

Account Closed The op doesnt state anything about the buyer moving in, so Im working on the assumption that he did not move in. 

I've added myself to some buyers lists with wholesalers in the areas we're currently interested in.

He probably then had the end buyer put down the earnest money, thus fulfilling his promissory note.