It isn't a problem if you don't mind selling all to just one person.
In my case, I've added a lot of people just to sell them lemons, they count on me and my in-game reputation depends on it, so I gather lemons through the day (even buy some if needed) and sell 3 lemons per market box [MBs] (I have 16 MBs) and each person can buy 3x a day (because it's too frustrating to be able to buy only 1 imo), so each can end up with 9 lemons.
It's not much, but I'm not their only source, so it helps with fruits requests during the day.
Now, doing this costs me a lost a lot of inventory space (usually 50 or 60 spaces, so I can refill the MBs a bit while people are still buying).
It's not much money for me in the end, but I'm there so I can help people play. It became a thing already, where they enter at midnight just to buy my fruits (it's my "midnight special") and then they enter again at 03:00h am. to ask for quarry entrance.
One person buying all my lemons means that while this dude got all my fruits and got rich, everyone else that entered that day, at that specific time, just to buy my fruits, found my MBs empty. Now, they don't know that someone cheated, it appears that I simply was not there to deliver yesterday (which is fine if I wasn't, but again, I sacrifice my gameplay to help others and have fun and this dude almost lvl.130 apparently it's better than everyone else. He don't even need the money).
There are the normal ones like peach or pear, and three specific ones that you are randomly chosen to get when you start the game: Lemons, Grapes and Lychees.
When you buy one that you don't have (in my case, grapes or lychees) and sell it to the game (not in the market box), you get a decent amount of money out of it, so it's good to buy some non-native fruits everyday just to make money.
Resuming:If it's not a game problem and he really did it, he's doing it for the money.