As with a lot of other DVD sets, your best supplier for selling on eBay is often going to be ... eBay. Pick them up in packages, resell them individually at a markup. Wholesale prices on DVDs, even from major outlets, tend to be so close to the price one can get at Amazon or other big sellers "retail" that it can be a serious waste of effort to try to start selling them new. But, particularly with baby & child-oriented stuff, the discs are going to be in good shape (because they're only ever handled by adults, in theory) and whole sets will not be hard to come by.
Frank