One of the reasons is that alot of the items at wholesale on our site are selling for the same price if not lower on ebay. Alot of items sell below wholesale value on ebay.
Try revising your searches to liquidations or overstock clearance items. Most of the time you can buy in bulk at 20% of wholesale value. This is also best if you are just starting out. You can easily double a 300-500 investment in buying liquidation items.
Sometimes prices on ebay allow you to compete selling wholesale items but in general the big powersellers are buying in bulk and getting much lower then wholesale prices.