The shopkeeper did not want to sell for____he thought was not enough

for what=that
he thought可以看做插入语
The shopkeeper did not want to sell that（ he thought） was not enough

对，插入语。是他认为不够的东西。他又没说是卖不够的东西，是他为了不够的东西而卖东西。个人理解是，他不愿为了不够的东西而做买卖，这个不够的东西很可能是钱啊。

that是真确的.