With no central currency, it’s hard to know what something is worth. Worse when someone wants to trade with you, you aren’t really sure you’re getting a good deal, or if you’re losing a lot.
I can say personally that I knew the Orb of Regret was worth something, but I didn’t really know what it was worth, but since I had no need of it, I left it in my shared storage. Boy, was I glad I did! That one is worth about 20 Chromatic Orbs, and definitely worth saving for future trades.
Do understand that when you see someone talk about GCP, that they are talking about Gemcutters Prism. So if they want the equivalent of 50 GCP for an item, you will be able to quickly figure out what you need to pay them.For me, that would be 2 Orbs of Regret, an Orb of Alchemy, and a Blessed Orb.
Hopefully the PoE Exchange Rate will help you to learn the information to know what things are worth today.
Keep in mind, the information is always changing, and as more people play, more items will become available, which could make for some great deals to be found for those who don’t know any better.