You can certainly ask, but according to e-bay rules if contact is initiated thru e-bay all sales must go thru e-bay which reduces your chance of getting a discount.
The seller can create a new eBay listing on the spot containing the quantity of items you want and a "buy it now" price total. Then you go in and bid/buy that lot.
There is a slight danger that some third party buyer might see the listing first and want that many of that item and buy the lot out from under you.