For outbound emails, do you use a company domain email ID or the domain ID you are selling itself?

I wouldn't use the name I am selling. If it gets a few spam complaints, then that will drop the value of the name.

As all of our domains are listed within our portfolio, I always email out using our ****@definitelydomains.com address. Its then in the subject header of the email where I list the domain I am selling.

Lots of similar threads—type outbound in the search bar. There was one going earlier this week.

I use the same email address and contact information listed in the whois registration for the domain in question. The best practice is to be consistent across all your communications and domain registrations. Find a domain to host for your portfolio and use a site like Efty to list all of your domains in one place. It's probably the quickest way to setup an online presence for outbound recipients to research the legitimacy of your email and company.