You can find information about the sellers simply by signing up as a government buyer. It's open to any government employee and should only take a moment. You'll then be able to access the full seller catalogue.
However, if you're specifically looking for pricing, because the Digital Marketplace itself provides a mechanism for rates to be set for individual briefs, we've removed pre-negotiated rates for services.
We found that fixed rates constrained competition and buyers are generally getting better rates through the Digital Marketplace than through the pre-negotiated rates we had originally.