Multiple bidder numbers are for the bidder's internal accounting, however they want to do things. They may be billed to different account names where the accounts are for different purposes, like resale, "check for stamp stock", funded by different investors, what have you. Then again, multiple bidder numbers might just be to try to throw others off that NYStamps is bidding on all those lots so people that think "If a dealer is bidding this much, then I can outbid them" won't always try to outbid them.
floortrader, I suspect that NYStamps has other places to sell junk lots outside of
eBay. They could easily sell on
eBay under a different alias to avoid tainting the "brand" name, but packing and shipping large lots is obnoxious and relatively costy. If I were NYStamps, I'd be breaking multibox lots down into smaller ones and putting them in small general auctions around the region. Load up a van and hit the road doing business along the way. Those small guys get anything up to some wild prices for junk boxes; I stopped looking at stamps and coins in general auctions because they were all junk and went for high prices.
I remember one individual collector that always used 3 numbers. One was for the collector's collection, the others were for two retirement accounts with different titles/names as they appeared on the auction bills. Since the buyer tended to be the ten per cent type bidder (where nobody else bid), I can only guess that he made out okay since he got at least a couple worthy lots every sale.