Having sold plenty of stuff on eBay before, PayPal fees are 3.5% (which is not that bad), but there is also 10% eBay fees charged monthly, so that is already close to 15% in fees; and then there is shipping which on a small package (think a game or DVD) could go as high as $8 or $9 priority in ConUS (Continental United States), even cheap shipping could still be $4 or $5, that is just a part of why people charge what they do on eBay.
If you go to large quantities packages, like a couple 10 packs of diapers; now you are at $15-$20+ for shipping
The other half being greed.
Point is if you can avoid eBay then sellers can give you the same stuff for at least 10% less; but either way buying multiple cases of diapers to be able to sell variety packs at $3 per diaper is a hefty investment all at once, even more so if the seller is to offer multiple sizes and all the different available prints.
Your best bet in small quantities of 1 or 2 diapers per design is an already established website like WearingClouds; otherwise you can either wait for a rare opportunity of someone having saved the last pack or two in a case for potentially years, order what you like in only 1 or 2 designs, or maybe split an order with another ABDL (preferably living near you to avoid extra costs in shipping).
But if anyone wants mixed packs of Barebums, Little Pawz, Kiddos, and Tykables Overnights, all in size large I can sell some mixed packs.