Do 3rd party seals like Truste or Verified increase signup/sales?

I would consider any study paid for by the company that benefits from a positive rating to be flawed.

Look at this from your customer's point of view. Have your customers heard of Verisign or Truste? If your customers have heard of them, what do they know about them? How much trust do they put in Verisign or Truste?

In my opinoin, the vast majority of consumers and most businesses have never heard of either company. So there is no difference for those customers in seeing a Verisign seal or one you made up called "GooberSafe".

I would take the Verisign case studies with a grain of salt. The case studies usually don't mention any other mitigating factors or metrics on those sales conversions.

I would give more weight into the TRUSTe slide deck. Goes into great detail on A/B testing and conversion rates.

Verisign Case Studies http://www.verisign.com/ssl/ssl-information-center/ssl-case-studies/

TRUSTe Slideshare Presentation http://www.slideshare.net/TRUSTeprivacyseal/how-to-use-privacy-seals-to-improve-privacy-practices-and-increase-sales

I've read a study about this subject (sorry can't remember the URL) and apparently it increases sales almost about 10% , it's any seal, it doesn't matter from whom.

That being said 99% of those security related seals are rubbish, one recent example. There are many other similar news.

I have no evidence to provide you with, but I suspect it will be largely affected by who your target audience is. With this in mind, I don't know that any study will greatly help you unless it is closely aligned with what you do.

A good example is a security stamp you get for SSL. If you are targeting tech savvy users, they will care about SSL and probably even care about who your provider is. If your users are the other end of the spectrum, they probably won't care much between an official stamp, or just a lock logo you find at write "secure" next to it.