If we sell most of our loans to ABC Bank on the secondary market and they actually underwrite the loan. We do the initial disclosures should we be listing ABC Bank on the GFE as a settlement service provider? Are they a required service provider?

Not in my opinion. The required service proider disclosur refers to Title Companies, Appraisers, Credit Bureaus, etc.

If your bank originates the loan and then sells to ABC bank, it is a secondary market transaction and ABC is not a settlement service provider.

However, if ABC is originating the loan and you are functioning as a settlement service, then ABC is the lender.