What does the actual contract of sale say? The Buyer cannot just choose what treatment he wishes.

Client list is an intangible, usually capital gain, usually long term. Buyer amortizes over 15 years. Seller reports gain over the life of the payments as installments.

If there is compensation, like a Consulting Agreement, that can be paid annually per the Sales agreement, and it could be an ordinary deduction to Buyer and Ordinary income to Seller. I said "could be" since it would have to be in a separate written agreement, separate from the Sale Agreement.

Absent a separate written agreement for consulting, it is a 15 amortization to the Buyer.