we do not share your data with anyone and there are many ways to control the visibility and storage life of your data here.
but the internet is a public space and you should always assume your data could be compromised.
be smart with your password, use tor or a vpn, and use pgp for sensitive communications.
The advantages to a buyer are.bigger than this. Not having a reputation sucks in case you have a dispute, couple that with not being able to write a review and you have thrown away your most important powers.
And for what? Having a username here does not change anything to your anonymity.