Box art - The Witcher 3: Wild Hunt

The Witcher 3: Wild Hunt [TIP GUIDE] How to Make Money Fast (VIDEO)

There are a number of ways to make money quickly in The Witcher 3 without using the PC console cheat (which won't help you if you're playing on the PS4 or Xbox One, of course, either). The Witcher 3 has a robust economy and sometimes certain merchants will pay more for easy-to-acquire items:

[PROLOGUE] - Sell Honeycombs to Tomira, The Herbalist

There's a sign on a tree in White Orchard that says that Tomira (who tells you where to find the buckthorn herb in the Beast of White Orchard Main Quest) will pay top dollar for honeycombs. You can find these by burning beehives using the Igni sign. Tomira pays 18 crows per honeycomb and 3-to-4 drop for every beehive you burn. You can take all her gold (about 600 or so crowns, as I recall) by bringing them in, in bulk. 

This FAQ will update as I find other in-game ways to make money easily (outside of gambling on Gwent, Horse Racing, or Boxing).