List broker
A vendor selling lists of email addresses — almost always a deliverability disaster.
A list broker sells lists of email addresses — typically organized by industry, role, geography, or interest. Despite still being a common offering, purchased lists are almost always a deliverability disaster for the buyer.
The problems compound. Purchased addresses haven't consented to your sending, which exposes you to GDPR/CAN-SPAM penalties. The lists are typically resold many times, so the addresses have already been hit by competitors and are saturated. Many addresses are spam traps placed deliberately by ISPs to catch list-buying senders; landing on traps is a fast path to public denylists.
The structural alternative — building a list from your own opt-in funnels — is slower but compounds. Every email sent to a purchased list damages the sender reputation you're going to need for the list you actually build. There is no version of list-buying that ends well; the math has been bad for at least a decade.