A Second Hand

You must be exalted with the Tillers and Best Friends with the various people for them to come and loiter around your farm. There would appear to be only two who are actually useful - Gina (vendor) and Jogu (crop predictions). I personally go for Jogu because I have a traveller's tundra mammoth.

[npc=58710]
He'll come and sit by the side of your pond. This makes it a little more convenient to ask him about the crop forecast.
Since he is only found at the market, he will come to your farm at any time, with the below exception.

[npc=58706]
She will stand next to Farmer Yoon and give you a repair vendor, as well as being the Tiller's Quartermaster with things like farm systems, vanity items, and goat mounts.
Since she is only found at the market, she will come to your farm at any time, with the below exception.

Edit: There seems to be an exception for the two npcs above - Gina and Jogu. If the npc is providing a daily quest that day, you are not offered the option to ask them to the farm. However, as long as you don't ask someone else to come to your farm, the next day that the npc isn't handing out a daily, they will appear on your farm once again.

[npc=57402]
He will come and stand next to Farmer Yoon looking not-so-decorative, and doing nothing much.
If he's at the market he will not come to your farm and you must wait for a day when he is at home.

[npc=58761]
She will come and stand by Farmer Yoon and look decorative.
If she's at the market she will not come to your farm and you must wait for a day when she is inside her father's house.

[npc=58707]
He will come and stand about on your farm with several chickens near him (he's feeding them, but not really).
If he's at the market he will not come to your farm and you must wait for a day when he is at home.

[npc=58709]
He'll come and sit by your sheep, and do nothing much.
If he's at the market he will not come to your farm and you must wait for a day when he is at home.

[npc=57298]
He will come and stand by the bale of hay and the tree stump, near the watering can, looking not-so-decorative, and doing nothing much.
If he's at the market he will not come to your farm and you must wait for a day when he is at home on his haystack.

[npc=58708]
She'll come and lurk on top on the bale of hay near the watering can.
If she's at the market she will not come to your farm and you must wait for a day when she is at "home" ontop of the hozen ridge where so many dailies are completed.

[npc=58647]
She'll come and wander about your farm looking at things (and doing nothing really).
If she's at the market she will not come to your farm and you must wait for a day when she is at home with her cats.

[npc=58705]
She'll come and stand just inside the pen where the yak and the pig hang out. She doesn't appear to do anything.
If she's at the market she will not come to your farm and you must wait for a day when she is at "home" on her pier.

Oh man, I seriously need a best friend. I am so lonely I started to talk to the shrine at my farm.

A second hand? Why doesn't Yoon get off his bear @$$ and do stuff? :P