Although you know what it's like to be short of cash, you're not doing too badly at the moment. In fact, sometimes your wealth can be considerable, and traders value a client of your stature.
Earning the Achievement
This achievement is earned by accumulating 100,000RU. It is directly awarded when the player has more than 100,000RU on themselves. The player will likely earn this over time, especially if they are an active artifact hunter. A good suggestion is to earn the One of Ours achievements, and to sell artifacts to Beard for the maximum price. Two runs through Zaton might earn the player the achievement before leaving for Yanov (although the One of Ours achievement requires going to Yanov).
By doing all Zaton missions and selling all non-useful equipment (including found weapons to Owl, excessive amount of medkits and antirads to either Tremor or NPCs and ammunition to NPCs), one may get this achievement even without selling any artifacts and leaving Zaton even once.
An exploit the player can use that works almost all the time is to get down to a very low amount of money (around 500 RU) and then ask a stalker to guide them somewhere for slightly more than what the player (i.e. a 450 RU-trip while the player only has 400). The player will end up with a negative amount of money, but oddly enough they will earn the wealthy client achievement. This is best done early in the game as the player won't have much money in the first place and so one can get the achievement and all its bonuses as early as possible.
A much easier way to achieve this however is to either get the Mutant Hunter achievement, or the Detective achievement, both of which supply infinite amounts of consumables in the personal storage of their respective area (ammo for Mutant Hunter, medical supplies for Detective). Simply locate a bed and sleep in it for 24h repeatedly and supplies will stack up. Once you are satisfied you may sell them to the trader who offers most. There is no need to stock up on food since you do not take damage from hunger while sleeping. Repeat until the achievement is earned. This is also useful for upgrading equipment or getting ammo that is hard to find anywhere other than traders.
- Hawaiian will sell higher-tier equipment if enough faction missions have been completed. With the A Friend of Duty achievement, Hawaiian sells rare equipment such as the Vintar BC. With the A Friend of Freedom achievement, uncommon weaponry such as the FT-200M can be bought.
- Owl will sell higher-tier equipment, also dependent on the player's faction progression. With the One of Ours achievement, rare weapons such as the Bulldog 6 become available, and other uncommon weapons (such as the Eliminator) become available with the Boss achievement.
If the player doesn't have any of these achievements, then A Wealthy Client will have no effect whatsoever, as it only affects the stock of traders when the player has enough reputation with a specific faction.