Let me correct myself on thumbtack.com you only pay if the customer contacts you. The quote below is from thumbtack.com.
it costs nothing to sign and you can advertise your services for specific areas. Say I want to test the market for my services in another town or area near me, so I add my service or services to that area and watch to see how many leads come in for that area and how much competition there is for that area or if I want to test out a new service like tile & grout cleaning, just add that service to my service list and add a target area and watch how many leads come in, if 10 leads come in and the competition is low for those leads then I can decide if it's worth jumping into that market, so far it has cost me nothing other than a few minutes to set up an account on thumbtack...Also with thumbtack I can try out various pricing models and find out which pricing model gets potential customers to contact me and if I set an appointment or have a discussion with that customer then I pay for that lead.
"What is a lead
A lead is a new customer who has seen what you offer and wants to learn more about working with you. You only pay the first time a customer messages you, requests a call or estimate, or asks for your availability. "