How's about this?
SELECT ID=MAX(Agent.ID)
,FirstName=MAX(Agent.FirstName)
,LastName=MAX(Agent.SecondName)
,Sold=SUM(CASE Seller WHEN Agent.ID THEN 1 ELSE 0 END)
,Bought=SUM(CASE Buyer WHEN Agent.ID THEN 1 ELSE 0 END)
FROM Agent
LEFT OUTER JOIN [Order]
ON Agent.ID=[Order].Seller
OR Agent.ID=[Order].Buyer
GROUP BY Agent.ID
:-D (And, yeah, be sure to index Buyer and Seller)