Dear Sleeps,
I am a bill collector. I work for a collections attorney.
First, let me suggest you do stop payment on the check because no doubt they will try and cash your check on the 28th. And if you know right now you will not have it, that is one fee from your bank you can avoid.

I will tell you, it is not uncommon for bill collectors to suggest having a garage sale, or trying to ask what type of assets you have in order to recoup their client's monies.
Although, she doesn't sound like she is a first party collector from your mortgage company. It does sound to me as if she is third party , perhaps hired from the mtg company.
If you are certain you cannot keep your house and it is about to go into foreclosure...then see about either doing a refinance to save it or put it up on the market.
You can also call your mortgage company and ask about a forbearance period with you only pay the interest.

I know how difficult this situation is for you. But, it is in your best interest to take action.
You can pm me if you have any specific questions.

I know bill collectors can be blunt and seem abusive to some. Unfortunately, we are not customer service reps and our only goal is to reclaim our clients monies.
I am here if you have any questions.
It would have been better if the woman you were speaking to had more tact.