Definitely you shouldn't register domains in bad faith to exploit someone's pre-existing brand name. That would be cybersquatting, and it's prohibited. The UDRP can result in the domain being confiscated on that basis.

That said, just because someone uses a brand name similar to your domain does NOT mean that they necessarily have any right to own – or prevent others from using – that domain. It's commonplace to see many companies sharing similar or identical names. And when the domain registration predates someone else's usage of a name, that other person generally has no rights whatsoever to the domain.

Trademarks are tied to a (relatively small) field of usage. So many different companies might share the same brand name but use it in non-overlapping applications or jurisdictions. And in doing so, they generally don't violate one another's trademark rights. In fact, the USPTO (which governs trademarks in the USA) often grants trademarks to the same brand name to multiple different entities simultaneously.

If you're selling a domain name, then it is the BUYER who wil be using it. And it's that buyer's usage that determines whether the domain would violate someone else's trademark or not.

For example, if "Tooth Fairy" is the name of a dentist's office with a trademark filed already, and your buyer intends to use "Tooth Fairy" in the same way, then they will violate an existing trademark. But if they intend to use "Tooth Fairy" to brand a video game, then they will not.

Usage is the buyer's affair. It's beyond your control as the domain seller what the buyer will do with the domain.

That said, if you are registering domains that contain – let's say – "Google", then that's clearly bad faith. Only register domains where there are legitimate non-infringing uses possible or many divergent potential buyers, no single one of whom can lay sole claim to the domain.