If you don't have best offer on your listings and the offers are not what you want for the items, just ignore the messages. You are under no obligation to reply, it is not as if it is a question about the item itself. Life is too short to waste too much time on unsolicited offers, most especially if they are heading into the insulting range.

Most of the time, I suspect they would be people who aren't dead keen on buying anyway, they are just trying their luck.