I think unless you are an established breeder or the sheep are pedigree, sheep sell for the most at auction as buyers see before they buy, or just take a punt if the price is right.
I've sold privately and its a always been a bit disappointing.. For a pair of nice crossbred sheep I'd probably try them for £80 each and then drop £10 a week until they have interest. You should sell them asap though as people have already put Rams in and if they add to the flock it will mean a longer lambing, so people are less likely to buy..