Bill raises cows. He has 3 cows, but wishes to eat chicken. 

John offers 2 chickens for 2 cows. Bill refuses as he would have 1 cow left – he needs 2 to reproduce. Bill offers 1 cow for 1 chicken.

Analysis. 
In part 2 we established 1 chicken to be equal to the value of 1 cow. The result of part 3 is still 1 chicken for 1 cow. However, the saturation point of the market is also 1 chicken and the market cannot sustain a second trade of 1 chicken for 1 cow.