If acquiring funds is a priority, don't sell to merchants!
This applies to almost anything you want to sell, but especially to dyes.

If you pay attention to the chat channels, you'll soon realize that dyes are like currency here, but I was a noob at the time, so I actually sold 1 to a merchant once for 1 lousy gold piece.

Hey, I didn't know. Now I do.

Always check with the dye trader in Ascalon City (post searing) for the latest dye prices. They change by the minute.

Black dye will bring you 7000+ (sometimes more) gold on the trade channel, but a merchant will only give you 1 gold for it! Silver, blue, green, red and purple are pretty popular as well, and commonly will bring 300-500 gold if sold to another player.

Stay away from that merchant when you want to sell!

If you really want to sell high volume, offer customers a bargain
Especially early in the game, where players' funds are limited.

If it's a dye you're selling, always undercut the dye trader by a bit. If silver dye is selling for 1,000 gold with the dye trader, offer yours on the trade channel for 700-800 if you really want to sell it. Offering it for 50 less than the dye trader isn't much of a bargain.

Give people a reason to buy it from you instead of everyone else.

Don't sell anything to other players until you know how much it's worth
Go to the merchant, the material trader, the rare material trader, or whoever has what you're selling, and find out the price first.

If the material trader is selling tanned hide squares 10 for 180 gold, that equates to 18 gold per tanned hide square. Think about selling yours at 12 or 15 so people want to buy from you instead of the material trader.

If it's a weapon you're loking to buy, go ahead and ask openly on the public channel what people think it's worth. You'll probably get a range and can go from there. If 3 people say a sword is worth 1.5k (1500 gold), but the seller wants 5k (5000 gold), pass it up until someone who actually knows the value of what they're selling comes along.