Bricklink allows you to add notes to a store and set them as your most or least favourite seller. To do this, go to a store’s page and click Add to My Favourite Stores. Then choose most or least favourite, or subscribed from the dropdown, add any comments you want, and click Add Store. Here you can choose to subscribe to a store announcements, (although in my experience most stores don’t use this feature) by checking the I want to receive announcements from this store option.
By adding a store as a most or least favourite, a star will show next to the store in search results (see below). This allows you to easily find stores you like when trying to buy parts from a wanted list. Additionally, if you go to Wanted > My Settings you can exclude your least favourite stores from showing up in search results.
Next: Buying from a store