Is it possible to reference back to the column you're selecting in the WHERE condition?
I'm not even sure how to ask this question without excessive verbage, so I'll just try to point you to the "rated_user = @2User" part and note that @2User is what I'm selecting.
SELECT id, user, minimum
INTO @1ID, @1User, @minimum
WHERE a_or_b = 'a' AND item = itemName
ORDER BY number DESC LIMIT 1;
SELECT id, user
INTO @2ID, @2User
WHERE a_or_b = 'b' AND item = itemName
AND (SELECT IFNULL(AVG(rating),0) AS Rating
WHERE rated_user = @2User AND completed = 'y'
) >= @minimum_seller_rating
ORDER BY number ASC LIMIT 1;