We Need More Writers!
You should almost never be losing gold on the trading post. Here are a few reasons why you are (if you are).
1. You're over-investing.
If you look at your balance and it's unsettlingly low, then it's going to be tempting to cancel a few auctions/bids in order to get back some liquid assets. One reason for getting low on funds is over-investing. Keep things on a smaller scale, especially if you're new to a market. You will lose a lot of money very quickly canceling too often.
2. You are afraid to wait things out.
There's going to be times when undercutters go crazy and you need to cancel in order to make a sale in the next week. But those times are rare, and more often than not, you'll want to just let the market play itself out. There are millions of buyers and sellers, so markets can jump up or down randomly due to unforeseen changes in either number for your item.
3. You post too many at once.
This is a huge problem that I see in emails almost daily. If you buy 250 of an item that only sells 50 per day, then you will really struggle if you post all 250 at once. You're creating a wall of items which other auctioneers will notice and undercut heavily. How do you see how many items sell per day? You can get an estimate using tools like Gold Wars 2.
Doing it Right
So how can you avoid those three mistakes?
1. Invest a small amount into each market you play.
2. Let markets go for at least a few days if things get dicey.
3. Post a small number of auctions each time, and increase the number only if those particular ones sell.