Toy with them as a cat toys with a mouse.
Tell them that you would be interested in whatever they're selling and then ask for some ridiculous sweetener - a Jag, a night with Angelina Jolie or Brad Pitt (or maybe both if you really want to throw them), 50 virgins etc.
Or answer the phone as a "special" person, a lonely old lady (it wasn't until I claimed that I had wet myself that one caller caught on) or a deaf old colonel ("What's that you say? Windows! Look here, m'lad, you haven't cleaned our windows for over 3 weeks and you won't get a penny until you do; and if I catch you so much as looking at my wife again I'll give you a sound trashing!")