I am not a breeder, but I absolutely love what my BCA's breeder provided with him. He provided a feeding chart and all of the usual pertinent info (snake species, parents, hatch date, etc.) At the top of the page were pics of both parents and the snake I purchased.

If I was to buy a het, I would want pics / info of the parents, hatch date of the clutch, etc. But also a pic to ID that specific snake. This will also protect you if someone claims you did not sell them a het (but is using a different snake than the one you sold them as "proof").

A statement on the same paperwork that states that it is 100% het with your signature would also go a long way.