Something I recommend is to take a look at what recipe the item is used in, and try to sell it in increments of how much is needed. For example, mythril ore is used to make mythril ingots. It takes 3 ore to make one ingot, so sell them in quantities of 3, 6, 9, 12, etc. Of course there's other examples like copper ore, which needs 3 for a copper ingot and 2 for a bronze ingot. In this case, you'd either want to sell in increments of 6, or sell in quantities of both 3 and 2.

There have been plenty of times where I've paid a higher price, just because I didn't want to deal with having a bunch of leftover junk. Which brings me to another point: Some folks look to buy in bulk since they want to mass produce something, and others want just a few for a synth or two. This is where it helps to know the item you're selling. If it's used to make something in high demand or a LOT of items (the core ingredients of ores, logs, textiles, and skins qualify here), chances are most customers are looking to buy in bulk. If not, then chances are you're selling to a niche crowd looking to make as few as one for their own personal use, so sell in small increments.