My only experience in buy backs was a client with a mortgage arm, about a year before the crash, had a meeting for appraisers. The CEO of the mortgage arm spoke about a loan in Bartlesville, OK they had a few years earlier. The price of RE is low and the borrower paid about $50,000. Unblemished credit history, and employed by the only game in town, Phillips Petroleum. Sold the loan to Fannie. Then Phillips 'merged' with Conoco. Frankly they sold out and were basically moved to Houston, stranding hundreds of people without jobs or any substitute jobs available. Housing prices crashed, the borrower defaulted unable to sell and he moved on to Tulsa and a new job.
Fannie then reviewed the original report and the reviewer found a nearby sale that closed for $38,000 or so. Demanded a buy back. The mortgage company and the appraiser said that they were aware of the sale but the condition was much worse so it was rejected as a comp. Fannie refused to budge, so they bought it back.
The moral of the story was that please explain any potential low priced nearby sales that are not used. I am not keen on "yes, we have no banana's today" but it made sense. The CEO basically said they considered Fannie Mae unreasonable, they supported their appraisers opinion, but that was the price for using Fannie Mae.