Crucial is the exchange of gifts with neighbors: choose neighbors and exchange with them frequently animals, fruits trees, that you can not buy. From level 7, you can sell on the market animals (gift or not, both).
If you aim to monetize,sell everything at once. There’s no sheep, pig, chicken or cow that count so much as a piece cultivated. But if you have enough money, you can use them as they were “decorations”.

About decoration: when and how many to buy?

The economic return of the money spent to buy fences, mailboxes, bicycles, wheelbarrows, and so on … is zero.
But they are very nice and helpful to beautify the farm.

The purchase of decorations should be postponed as long as possible. Level 3 costs 500 coins do not weigh on budget as 500 coins at 19, when your current revenues are much higher. Neighbors: friends and relatives to help.
At the beginning you often visited the neighbors, earning 20 coins to fix the field when they are not connected is convenience. Especially if just one click. Exchange a gift at least 48h with each neighbour: usually who receives is happy to respond likewise.