An "old lien" could be anything so is it safe to say that the buyer's who had the property under agreement thought that it was too difficult/expensive to resolve the lien issue?

If you look Renatus up on BBB (if you believe in BBB) you will find the that they have received only five complaints, only two of which were not resolved to the customer's satisfaction.

For example, I just bought a kwikset "Premis" smart lock the other day (fairly new model in their smart lock line,  similar to the Kwikset 913, 914, 916, but has the touch screen instead of buttons and this flavor is compatible with iOS, instead of the other protocols like ZigBee or Z-Wave).  

It sounds like you have a great opportunity, I would stop talking yourself out of it and pull the trigger.Also when you talk to them ask them why they are leaving and see if you can help them resolve the issue so they stay.

I am curious, how did this all resolve?  

Fortunately, Justice of the Peace Courts (the "People's Court") handle amounts up to $10K so it's pretty easy to file an action for less than $150 and it can usually be resolved in less than 60 days.

Being a landlord takes patience, flexibility, strong resolve when enforcing rules, and a long fuse.

It'd be tough to get the "best of both worlds", since the best of the single-lease-world is charging utilities, and less time spent managing - neither of which are particularly compatible with a student rental property. 

I would try to resolve their concerns but ultimately this is not one I would push.

I will get her to consult a lawyer to see how this issue can be resolved.