Well if people are that bothered about being offered £20K why put a post up that says "serious offers"? Why not "offers over..." or "offers in the region of...".

An offer is an offer, the seller can only say yes or no, there's no need to be rude about it. I'm sure the guy offering £20K is serious about it thus it is a serious offer, if it's not what the seller is looking for then so be it, there's no need to have a go!