Depends on the merchant. If it's one people are constantly buying stuff from then he'll almost always have cash on hand.

Here's my advice:

1. Look for "high dollar" items. Typically these will be magical things like scrolls, wands, potions, etc.

2. Once you've found some of these, try to find multiple merchants who'll buy them. Most towns have some sort of "magic dealer" guy.

3. Note that most "magic dealer" guys don't buy scrolls. Typically there's a separate "scroll dealer" guy.

4. Note that not all merchants are located inside cities.

5. One way to "make" money is simply to "save" money. Barter when you can. Try to find the cheapest versions of return potions, teleports, etc. (Sometimes "cheapest" = "free").

6. Try to get up to the commerce skill that juices your ability to haggle. This will usually only happen later in a character's life, but it's pretty potent. I think I was buying return potions for 1/2 the normal price. You also get more money in return from merchants when you sell stuff.

7. When you get up to approximately level 40, depending on class, there are usually easier ways to accumulate gold than gathering and selling stuff. But you'll have to find those out IC.