A player asked on Hay Day Reddit, "What are bot farms and are they legal?" And here's my reply.
First of all, per Supercell TOS, players using bots to play Supercell games are violating the TOS and may be banned from playing.
Some players use bots to wheat nonstop so they may get the drops of Upgrade Materials (UM). Some players take it a step further by selling UMs on the black market in exchange for real monies.
Then there are the criminals belonging to organized crime who are selling UMs as one of the ways to launder monies. I think that's something to consider if we buy UMs from a website run by organized crime.