There are quite a few. Unfortunately there is nothing stopping the developers from registering another account and reviewing the items they have for sale themselves. 

Having said that these are quite easy to spot. For example any new plugin with more than 3 positive reviews published within a month indicates that the reviews are most likely left by developers themselves. There are a few notable exceptions but you can use this rule of thumb to weed out the fakes: 

1. Many positive reviews and at least one negative 

2. Only 5 star reviews within a short period of time i.e. 1-2 days after the plugin is published

3. Only 5 star reviews and a lot of unresolved issues in plugin forum

To me, if they do not put the URL to their unique domain it feels suspicious.

I will always advice anyone purchasing a plugin you are not so sure about to go through the forum topics of the plugin before purchasing. And of course, it wouldn't hurt to make a small pre-sale enquiry.

I purchase using PayPal, and it makes it easier to get credit for a plugin that fails to work.