Tim values treat for his dog at $10 per box, and John values them at $6 per box. If the price of dog treats is $3 per box, but only one box is available between these two buyers, then gains from trade will be maximized when:
a. Tim buys treats.
b. either buys the treats since they both value them more than the market price.
c. consumer surplus is equal to $3.
d. John buys treats.

Consumer Surplus is the difference between the price he pays , the maximum price he could have paid for the product. Graphically, it is the area above the price , below the demand curve.

Producer Surplus is the difference between the price he sells at , the minimum price at which he could have sold the product. Graphically , it is the area above the supply curve , below the price level.

In this Case : Since Tim values dog treat more, he would be having more willingness to pay (lets say = $10). So , the consumer surplus = $10-$3= $7 .

Since John values them lesser , he would be willing to pay less (lets say=$6) So the consumer surplus = $6 - $3 = $3 .

There is no change in producer surplus - prices are same , nothing about producers' minimum sale inducing value.

So , considering only given consumer Surplus : It will increase by $7 if Tim buy treats, It will increase by $3 if John buys it. Hence, purchase by Rim is better (given only 1 box) as per perspective of benefit of trade.