On the SiteOne request account page the choices are "Contractor" or "Homeowner/Retail Customer". I requested the homeowner, my request had to be reviewed for approval (the request didn't ask for my financials), and about two days later I received an approval email with login instructions. After I login, I see the product prices, and filter select for what's available at my local SiteOne location.
I'm not 100% sure on the remaining purchase procedure since I haven't bought yet. My online account info shows nothing about having store credit so presuming I purchase using my CC: Select what date morning/afternoon pickup timeframe I want, then go pick up item(s) on the requested date at the SiteOne location, pay with my CC.
Several help videos here: https://www.siteone.com/en/help