You can, and nothing stops you. Most sellers do not, because a lab buys through procurement from counterparties who warrant the rights, handle the personal data, deliver in a shape it can ingest and still exist in a year.
What a lab asks a direct seller for
A rights warranty across the whole corpus, a written position on every kind of personal data inside it, an indemnity, an export in its format, and a counterparty it has already onboarded. Producing that for one company usually costs more than the deal.
What changes through a broker
We hold the buyer relationships, the paper and the delivery. You answer the form, sign one licence and provide samples under NDA. Buyers see a codename until a buyer signs. You are paid out of what the buyer pays and pay us nothing.
Exclusive or not
Direct deals are often exclusive, which prices the records once. A non-exclusive licence lets the same records be licensed again as they grow.