My stock response is "Ask the customer what he's doing." I'm reluctant to do that here because I'm pretty sure that the attorney is doing something of which Hoyle would not approve.
As indicated by the fact that they are ineligible for exemption, auctions can very readily accommodate money laundering.
If the auction was selling something for the client, he could pay the client directly. Instead, he's paying the lawyer. If the lawyer was just a straw man for the client, he could write the client a check. Yet, by buying the official checks, the lawyer may believe he is eliminating a paper trail to the client.
Learn a bit more about the auction house and the attorney's practice and put it in your stack for SAR consideration.
Wish I could help you investigate and write! Story
about how attorneys sometimes get involved in their clients' business activities.