In my experience, Thumbtack is little more than Craigslist. I've sent out maybe 10 offers via Thumbtack, and all but one ended up with the client deciding they weren't serious about the project and not hiring anyone. Thumbtack charges a few bucks per offer you send (which has slowly risen, the first one or two were free, then $2.99, then 5.99, the last one was 8.99, or you can give them 8% if you get the job, which is frankly nuts), which isn't a big deal because this is their refund policy:

Individual Leads Refund Policy
Thumbtack will refund your payment if either:

The customer did not respond to your quote either by phone, email or Thumbtack message 7 days after you send it
The consumer was not serious about hiring any service professional

Individual Lead purchasers can request a refund by emailing Thumbtack customer support.

That is their written policy. However, recently, I requested a refund and got this response:

"We will fully refund your payment - no questions asked - if you don't hear from the potential customer within a week after sending your message, if you learn that they never intended to have the job done by anyone, or if you learn that the job was done by someone outside of Thumbtack.

Unfortunately, we are unable to issue a refund on this job: (title) since the job was completed by someone from our site. I'm sorry about that."

I Googled that policy. It doesn't exist.

The phrase "done by someone outside of Thumbtack" came up four times, all of them on their support forums, from the same customer service person, claiming it was policy... but it isn't written anyone.

I challenged them, told them plainly that it isn't policy, and they quickly told me I'd be getting a full refund.

I've heard similar unsavory stories of Thumbtack crossing their stories, but I found this rather unacceptable. Be careful out there.

Thanks for the warning. My experience with their charges slowly rising (the last one today being 8.99) has been similar to yours. I think that you are right about being careful.

Never heard of them before.

It would seem to me that the 8% cut would be the way to go... sure, on a $500 day job, you have to cut them $40... but then you don't have to give them anything for all those jobs you don't get... they only get paid when you get paid, and you can figure their cut into your bid fee.