They don't have to get the highest price possible.
Say the absolute best price of a property was £200k. If the mortgagee puts it in an auction with advertising, marketing etc. and gets £190k there isn't going to be any issue. If they slap the property in an auction to get shot of it quick, and only get £150k, they might have questions to answer.
Well they don't have any control over the price in the auction, so long as they tick the boxes re: being able to show they marketed it then... Beyond that the auction is a quick and easy sale for them - the outcome isn't thier fault if they've met the typical requirements.