certainly you can make fg from ls e.g. by selling good items, mutables you find during your climb, from items you buy cheap and sell higher or some guilds who also pay for guild points you get.
but usually, you have to invest a lot before you make profit. you can play the game without spending any fg, but it is a lot slower and takes more time to kill the same amount
of monsters than you would need with a better equipment. many players play the game for fun mainly and they will certainly never reach the profit numbers because they keep investing into new gear
a very important aspect is to have knowledge about the game itself, this helps to make the right decisions concerning gear, the way to play and trade.
this one might help you to get an idea of financial fields:
http://forums.d2jsp.org/index.php?showtopic=18427600&f=272additionally, you can find a lot of information in this collection:
http://forums.d2jsp.org/index.php?showtopic=18560908&f=272on the one hand on specific character guides, but also about game mechanics and more.