There are a few ways to locate the person's profile, if the ad is still up, or if the person has other ads up, that's the easiest way.
If not, do you know the person's username? If not, maybe just an email back to the member to ask them their username.
If yes to one of those options above, then this article has pictures and description to help you out. https://www.gunpost.ca/tips-selling#Credibility
Now if you don't know the username, or cannot ask the person for their username for some reason, then feel free to send me their email address and I can look it up for you. (But keeping note of the person's username or keeping a screenshot of the ad is a good practice for future dealings.)