the man , known only as xu , was keen to have a grandchild .
he bought the ` daughter-in-law ' for £ 1,311 at the beginning of 2014 .
both his son and the woman that xu bought suffered from learning difficulties .
after realising the young couple were not sleeping together , he began having sex with her himself .
but when she failed to fall pregnant he decided to sell her on again .
xu is under arrest for human trafficking at lianyungang in eastern china .