Surprised drug trade is even being talked about, still.
The way OB was designed was to prevent the identities of others from being able to be discovered by law enforcement (or any other central agency/authority) in an efficient way. This point is that it is supposedly more safe to buy and sell illegal items using OB.
The case for OB is really, really clear. Go
here, think about some things around your house, maybe something you make for a hobby or business, and see what Paypal/eBay'd charge you.
Just to throw out some examples (fixed price auction, no add-ons) for the exceptionally lazy:
$6.99 jams & jellies business = 21.46% fee per individual unit, 14.33% per six-pack
$10 Playstation code = 18.9% fee per unit, 14.1% per $50 card
Fujifilm "Instant Minis" = 16.12% fee per 20-pack, 14.34% per 50-pack.
eBay was really not designed to be used to sell these kinds of low value items on it, although many people do in order to increase their potential number of buyers. If you were to include the cost of shipping items in a way that can be tracked (which would be necessary for both OB and eBay transactions as if this was not used then the buyer could always claim they did not receive their goods) then the total cost would be much higher. eBay was really designed to sell higher end type goods that would sell for several hundred dollars.