There are a few ways to locate the person's profile
If it was a buyer, or someone who inquired on your ad. That first email that you got from the member through the site has his/her profile name as a clickable link just under the GunPost logo. You can click that and it will take you to the profile where you can leave a review at the bottom of the page.
Or, if it was a seller, and if the ad is still up, or if the person has other ads up, just click the 'leave a review' option in the message member box there.
If not, do you know the person's username? If so, you can search it in that link in the footer of the site that says 'Active Members'. There is a keyword search box there, where you can type a profile name and pull up matching results.
Another link that could 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.)