They definitely aren't selling your 300k mortgage for effectively 50 to 100k if the assumptions you make hold (e.g. there actually is a 300k house as collateral

They definitely aren't selling your 300k mortgage for effectively 50 to 100k if the assumptions you make hold (e.g. there actually is a 300k house as collateral), so the whole premise is flawed. First, it seems they're selling at something like 85%, so they're selling a $300k mortgage for something like $250k. Second, the main reason they're selling because these collaterals have fallen in value and so it now would be much less than $300k. Third, any $300k mortgages it would be possible for you to get financing for the $250k simply would not be sold as part of such a discount package (because those are likely worth the full $300k or close to that), the discounted sale gets those loans for which they (knowing all the data) know that an equivalent financing won't be granted in this market - because anyone who'll grant you a $250k financing will rather buy the same thing from a lender for more than $250k as part of a large scale package deal with less overhead; refinancing is a reasonable option only if they can lock in a much better interest rate and thus make it more profitable.