Wealth in Diablo 2 is a relative concept. To many (probably most) players, wealth doesn’t refer to the amount of gold sitting in your stash. It instead refers to equipment, gems, runes, or other miscellaneous items. There are many techniques used to increase one’s wealth and become “rich”, but here I will simply cover the basics.
The most obvious technique a player could use is going out into the wild, killing alot of monsters, selling whatever they drop, and then repeating the process over and over again. While this is effective for new players, there are some ways to improve it. For example, try to avoid picking up large items like spears or longbows, as they take up quite a bit of inventory space and don’t sell for much. Instead, try looking for items that are class-specific, as these often have skill bonuses which make the item worth much more. Also, be sure to pick up (and identify!) amulets, rings, and charms, as these are often worth a significant amount of gold. Same goes for the throwing potions.
After a player has built up a decent amount of wealth, he/she could try Magic Finding (also known as “MFing” – a terrible acronym, if you ask me). Magic Finding is essentially the same method mentioned in the previous paragraph, except here the player wears equipment that raises his/her chance of finding magic equipment (“+X% Better Chance of Getting Magic Item”). Boosting this chance by 150% - 200% is usually high enough to make a significant difference in the items dropped from monsters. You can increase this chance by wearing socketed armor and putting topazes in it. Not only will Magic Finding allow you to grow richer more quickly, but you also have a higher chance of finding rare, set, or unique items that you may wish to use yourself.
There are many other methods to gain wealth, but most aren’t considered good practice. For example, there are websites out there that sell Diablo 2 items for real-world money. Also, if you are playing offline, there are mods you can download and use to change your character stats and give him/her any item or amount of gold you want. I don’t promote such methods - to me personally, they take away the challenge of the game and it makes it less entertaining. But, I thought I would mention them just to be thorough.
Ultimately, if you want to increase your wealth, then Magic Finding is the way to go. It works for any class and is especially effective for boss runs (by the way, if you’re looking for runes, the Countess drops them quite often). Increase your chance to find magic items as high as you can, then make boss runs wherever you happen to be in the game.
Happy hunting. :)