Since our server is new, the market is growing quickly by the hour. As players get more loots and sell them, their purchasing power increases, and the total zenies exist on the server increase.

So at this stage, prices of rare items are always adjusting due to "inflation" (more money in the market). Also, people are "zeroing" in on what the prices should be.

One tip I can think of is using amity guild's market watch website. They are part of anther RO server, so their item worth is different from ours. However, I believe we could leverage the comparative worth of items.

So If you are not sure how much your rare loot is worth, Google "item name amity". For example, if you just got a creamy card, and you are wondering how much it's worth. Type in "creamy card amity" in google, and you will come to this page. As you can see, on their server, creamy card is around 20m. If you noticed, several sucessful trade of creamy card in our server has been around 1m. Therefore, you can estimate that their prices is 20x ours. This "ratio" is rough and can change on the daily. BUt if you see creamy card selling for 20m, and another card selling for 40m.. then you should consider the other card worth twice as much on our server.

TL;DR: Google "item name amity" to see relative prices of item in RO.

PS: I'm no economist, just trying to figure this out. If other RO experts find major flaws in this estimation method, please let me know. Again, this is a reference website so you shouldn't be using it as a bible anyways.