Word for someone who promotes a product/idea that he/she is invested in

Solution 1:

biased
ˈbʌɪəst
adjective
unfairly prejudiced for or against someone or something.
"we will not tolerate this biased media coverage"

(Copied from Google's definition of the word)

Or a person promoting their vested interests

a special interest in an existing system, arrangement, or institution for particular personal reasons; the persons, groups, etc., who benefit the most from existing business or financial systems

(dictionary.reference.com)

Solution 2:

Another good word is shill. A shill is someone who promotes a product, for instance, leaving a review on Amazon, while secretly being affiliated with, or paid by, the sellers of the product.

From dictionary.reference.com, second definition:

a person who publicizes or praises something or someone for reasons of self-interest, personal profit, or friendship or loyalty.

Solution 3:

In the technology adoption arena, the term evangelist applies.

https://en.wikipedia.org/wiki/Technology_evangelist