Artisan Profession
As mentioned, the 'Artisan' profession (Farming Level 10) increases the value of all artisan goods by 40%. This is arguably the most profitable profession in the game.
Aging Casks
Once you upgrade your Farmhouse to the third level, you unlock the Cellar, which can house Casks. Casks age Wine and Cheese, increasing their value significantly (up to double for Iridium quality). This is a slow but highly profitable passive income source.
Truffle Farming
Pigs (from a Deluxe Barn) find Truffles daily when outside. Truffles are highly valuable, especially when processed into Truffle Oil with an Oil Maker. The 'Botanist' profession makes all foraged Truffles iridium quality, further boosting profit.
Fish Ponds
Fish Ponds allow you to raise fish and harvest their roe, which can be processed into Aged Roe (caviar for Sturgeon roe) in a Preserves Jar. Certain fish (e.g., Sturgeon, Lava Eel) produce very valuable roe.