It's very much serious. The buyer's party is located in Orange County, CA and the funds are at Signature in NY. The third party platform is is similar in size to Gemini, however that will be disclosed further into the process.
What third party ? Did you mean escrow?
Looking for large quantities of Bitcoin at a 10% discount.
I doubt that anyone will be going to sell their bitcoin in discount price than the current market price. You should be trying with x% higher than the current market price because here you are the buyer in need.