What is the ruling on a Muslim being rewarded for the intention of bringing joy to a seller when making a purchase, and what other good intentions can he call to mind?

A Muslim's diligence in having a good intention for permissible acts is a good deed, contains much good, and a Muslim is rewarded for it. Examples of this include: intending to bring joy to the seller, and intending to benefit from permissible things in obedience to Allah. This is what some scholars call the "trade of the shrewd and intelligent."