The irony of the "TrustPilot" name is there for all that want to look for it.
If you post a negative review based on your experience, after a little while they will remove it. You will be sent an email to provide proof that you are in fact a customer. This in its self seems fair enough and you could argue that it does indeed improve the trustworthiness of the site as it would stop competitors posting false reviews.
However, when you provide this proof you will receive no acknowledgement, either positively or negatively and after 7 days all trace of your review will be removed.
Take for example, Savi.co.uk, a website currently scamming people with EPC's. They do not provide a service, cannot be contacted, when you try to cancel you get charged for nothing and given just 24 hour to pay before they start adding £15 surcharges to this cancellation fee. They have many negative TP added every week and within hours these are censored by TP and removed under the no proof of customer policy Go have a look, you can see this happening for yourself. This leaves only positive reviews. To add insult to injury, check out the number of reviews posted previously by these reviewers, 2 at most. Its not too much a leap to suggest that not all these positive reviews are real.
All in all, it would suggest TrustPilot is very much not to be trusted. I strongly suspect we will see it shut down by Trading Standards or exposed by the press in the not too distant future.