i also believe eBay is good for determining values. However, it's important to see what they are SELLING for, not what they are ASKING for.
I see guys at fea markets with games. When you aks them how much, they wip their phone out and look it up on eBay (which pisses me off). Than they try to show me something with a Buy Now price. I tell them to show me something that has SOLD for that price.