The estate agent was there longer than any prospective buyers. Are you sure they aren't BSing you?
Constant smell of weed is a pure exaggeration, so best to remove the emotion from your post.
I don't think so as have had the exact same problem.
My old neighbours who have moved now thank the Lord (did a runner in the night leaving the home owner with a mountain of **** to get rid of) used to smoke weed in their garden very regularly throughout the day. Every day I'd come home to the smell of it in my house despite windows being closed. It was embarrassing when I had visitors.