I noticed I can sell things at a market type thing by my farm. Do only my friends have access to that stuff to buy, or can anyone buy it? (I don't have a lot of friends, so I want to know if it is worth putting stuff up there, if they're the only ones buying it.)
Stuff you put in the Market is available to all other players, so even if you have no friends (sniff) it's still worth putting stuff up there for sale.
You can also buy from random strangers on the internet if you lack friends (or a co-op) to buy stuff from. And unlike actually buying real fish from the internet, it's always a safe transaction!