One factor that nobody has mentioned is timing.
When a customer goes to a retail/online store to purchase something, it's at a time of his choosing.
When you try to sell a one-off, you're setting the timing, which dramatically reduces your customer pool.
Neither ebay prices nor retail prices constitute valid price points if you're trying to sell something NOW.
Ebay prices are only a valid indicator for common items, not for unusual, scarce, or unique items.
A couple of years ago I saw a $100 radio sell on ebay for over $4,000 due to a bidding war between two idiots.