If a customer deposits a reportable amount of cash and we believe it's suspicious based on any criteria, including the amount alone, we're going to file a SAR. The fact that we filed a CTR already doesn't excuse us from SAR filing. I'm guessing your customer isn't able to or required to file SARs, and I'm guessing they aren't inclined to call up their local law enforcement to report suspected money laundering on their customer.
Whatever your decision, you need to document how you came to it. If you think it's suspicious, file. You can decide if the 8300 form helps mitigate whether your customer is a subject or not, but even if you decide they aren't you should still consider whether a SAR is warranted with the buyer as a subject or no subject identified.