Kelly's guide is pretty much the best way to find out values.. but if you can't find the items there and don't want to make a fair trade board on the NC I can give you a rough estimate for item values :3
Most items that aren't on the list are usually still buyable items or items that are still obtainable and not worth 1 gbc. Atleast that's been the case for some items I thought were nice but turns out they are just common items.