She tells me I need proof of funds letter.

Some banks all they need is proof of a security deposit, maybe first/last months rent along with a copy of the lease agreement, and can be counted.  

Usually showing proof of funds or having a reference (title company) usually does the trick for me.

Used available credit as proof of funds and transferred the cash into your bank account prior to closing.

Lenders, like ourselves, exist that do not require proof of personal income.

Sounds like this deal isn't going to work for you after all; I just wanted mention that when /if a "cash" offer is accepted, you will likely be asked for proof of funds right after.

(I knew it was a long shot but thought I would try)  I am hoping to send them an assignment of mortgage as proof so I can be added as additionally insured.  

And proof that we really can start up a new career/business in our 'spare time', which is what I've been working towards.These seem like mostly cash deals, so I was wondering where that was coming from, if you were doing hard money or something.

Prefer them to be class A or B, with experience, references, proof of work and in good standing with all accounts.

Yes, there will be buyers who walk - but the best way to combat that is to show proof of all inspections passed.