This is just my opinion, but I don't care if you are 14 or 84- it is getting too difficult to sell coins online. If I did sell online again, it would not be thru eBay. Why? The chances of you getting ripped-off by: 1. ebay, 2. paypal and 3. the customer, are very likely. What I mean is, you can be ripped-off by the TOS (terms of service) that you agreed to just as easily as with a gun. But this doesn't really answer your question. You've looked at eBay coin prices. The majority of eBay sellers coins are higher than most pricing guides/books. See below for why eBay prices are higher.
How would I sell coins online? First, I would sell coins thru my own website with email address. Then you need to decide how to get maximum exposure for your website. You need to decide how you will handle monetary transactions- credit card? personal check? bank/cashiers check? money order? Next, I would determine what kind of coins I wanted to sell- new? used? slabbed? The biggest issue with coins now is counterfeiting. Your customers will want some kind of guarantee or assurance that the coin you sold them is genuine. After that, I would begin looking for several supply sources- banks, yard sales, coin shops, flea markets, etc. Remember, you want to BUY low and SELL high(er). You will need a camera to take pictures of your coins for sale on your website. You really need a phone so your customers can ask you questions. You need to decide how you will ship your orders- Post Office? UPS? FedEx? You need to determine shipping rates- Exact or Flat rate? Insured? Signature Receipt? You want to ship like a pro- don't ever ship a coin in a standard envelope with just a stamp. All of these options add more cost to your (potential) sale. Finally, you need to decide how to handle Returns. No Returns? Returns under certain conditions?
If you still want to sell coins after reading this, I wish you well in your endeavor!