The Mets’ best opportunity to sell low on Matt Harvey came on Dec. 1, when they could have washed their hands of him and bid him farewell for the low, low cost of zero dollars. They passed, tendering him a contract and thereby committing more than $5 million from their incredibly shrinking payroll. So now that they’ve re-upped with The Artist formerly known as The Dark Knight, they might as well stick with him, right? In for a penny, in for a pound? What’s the point of selling low, after all, when you could have sold even lower and saved yourself some effort? The Mets have engaged several clubs on Harvey, The Post’s Mike Puma reported, though they expect the right-hander to be a 2018 Met. There’s no harm in listening, naturally; a well-run team stays open-minded, just as the Yankees entered this offseason with no intention of touching their offense and wound up adding Giancarlo Stanton.