From experience, I guess you'd struggle to pay them enough. A decent salesman can earn £30k a year, a good one over £50k (not to mention perks like company cars, pensions, etc).
I don't know how you would make it attractive enough to get someone who would make a significant difference to your sales (unless your not talking about them selling your carpet cleaning services?).
The window cleaners use canvassers, and I think they pay them the equivalent of the value of the first clean - but of course this is recouped from ongoing custom. Also, its much lower value and quicker to talk someone round to having their windows cleaned as opposed to their carpets.