Seller trying to renegotiate price after buy now used

Legally, if that was a genuine mistake, yes they can do that under the contractual mistakes act. Of course "genuine mistake" is the important words and then the hard bit is to prove in is genuine or not. it would however be looked at in pretty bad tate though.

Absolutley, as L says. However reporting them to TM maybe a waste of time as if it was a genuine mistake, by not completing trade at the original price, they are following the law of the contractual mistakes act, and TM authority is not above the law. But it maybe good for TM records if this was an on going recurring situation

Sounds very much like an honest mistake as the seller has said, in which case you can't hold them to it. If you don't want at the new price the seller has suggested, let them know and continue your search.