are all these people giving lists of 10k+ leads trustworthy?

I'd recommend to verify and clean the list before you use it.

No, because people by default can not be trusted :) what type of leads are you looking for and what type of product or service are you selling?

when working with massive lead lists it’s always smart to verify them first otherwise a lot of emails might bounce or be outdated tools like zillion verifier can quickly check large batches and give you confidence that the contacts are real and deliverable