When you want to sell items, as far as possible, sell at the Yavin Space Station - Suvam Tan will pay more for your items than any other buyer in the game.
It should also be noted that Suvam will give you a discount on his goods if you beat him enough times at Pazaak - handy as he sells some of the best gear in the game.
Relative to Pazaak, although it is true that luck comes into the game to an extent; you can help tip the odds in your favor by buying up as many +/- Cards as possible. The exception is on Taris where the seller Uriah has an unlimited stock of +/-6 Cards - obviously you can't buy an infinite amount, but buy a few of them.
Once you have enough +/- Cards, you can completely fill your Deck with them; this guarantees that the 4 Cards, which are randomly selected for the game, will all be +/- Cards. +/- Cards have the massive advantage over simple + or - Cards that they give you 2 options each in terms of altering the in-game number total.
In general, lower numbers on the +/- Cards are more useful; hence why +/-1 Cards are generally more expensive than +/-2 Cards; which in turn are more expensive than +/-3 Cards, etc.
Other than that, simply play a lot of Pazaak and you should start to get a sense of the tactics; such as when to hold, when it's worth playing a Card, etc.