Is it possible to find the user whose aggregated score reaches X value first?
First you will need a table with a column representing how many sales a seller has accumulated at a certain point in time:

SELECT 
  `sellerId`,
  `saleDate`,
  ( SELECT SUM(`saleValue`) 
    FROM `mzarzycki_test_sale` b 
    WHERE b.`sellerId` = `a.sellerId`
    AND b.`saleDate` &gt;= '2001-01-01 00:00:00'
    AND b.`saleDate` &lt;= a.`saleDate`
    GROUP BY `sellerId`
  ) AS `saleSum`
FROM 
  `mzarzycki_test_sale` a
WHERE
  `saleDate` &gt;= '2001-01-01 00:00:00'
GROUP BY
  `sellerId`,`saleDate`
ORDER BY `sellerId`,`saleDate`


The rest is easy:

SELECT SaleDate,SellerId FROM
(...) c
WHERE c.Salesum &gt;= 30
ORDER BY c.SaleDate
LIMIT 1


SQL Fiddle