"Guessing if there were laws against them selling the goods..."

Well there IS with Treasure, hence the reward system.

Here's a funny thing though, Treasure items all belong to the Crown and all other items belong to the landowner - so unless the landowner has specifically handed them over as a gift no metal detectorist has ever dug up, taken home or sold any item that belongs to him.

They do show everything to the farmer though. Obviously.