I generally try to compare everything I sell to other things on the market, which, you're right, that can be hard when there's huge price variances on an item.
I also always try to think what I would offer if I was buying the item.
Other "rules" I follow:
If I bought something used, I generally try to get roughly the same price I bought it for, unless I've had it a long time, I beat it up somehow, or a newer version was released.
If I bought something new and there's nothing to really compare it to on the market, I'll usually just take like $25-$100 off the new price based on the original price, how long I've had it, newer versions, etc.