You need to start with a niche and a community.
That means a geographical restriction and product restrictions.
Good candidates for a niche include things that people only buy once or intend to buy once where the cost is high and people can afford the time to do their research.
You also need your niche to be something people are passionate about and therefore willing to put in the effort writing reviews.
Once you have momentum in your niche you can then accept reviews for a wider range of products. Then, if that goes well, you can go to other markets, for instance open up in the rest of the English speaking world.
Facebook started with one college, went on to other colleges, went on to the colleges outside the US and then opened their doors to everyone.
Gamification works too. If you can get people in a trade to be motivated to write reviews then you can give them internet points that they might like to have.
Also try and work on your own pain points. What have you agonised over buying to arrive at a choice that has worked for you?
Products also need the resources to keep them going. Service manuals and part numbers matter.
How people use the site matters too, for adding in reviews you almost need a complete product database of every product ever made by barcode with GTIN images magically appearing.
If you can engage people to submit reviews, e.g. this week we are looking at toasters, what you got folks? as the topic. Then people could be prompted to help out.
As mentioned though, the scope is considerable, larger than an ecommerce project and you need to focus on a niche, get that working and then use the platform for everything else.