A Purchase Contract and a Sale Contract in AgriDigital are the same thing, it just depends on your perspective of the contract as follows:
If you are the Buyer of the contract, you view it as a Purchase Contract.
If you are the Seller of the contract, you view it as a Sale Contract.
A Contract can be created to buy or sell, so the creator of the Contract has no bearing on your perspective of it being a Purchase or Sale Contract.