I'd say eBay is a good idea, as it really is a seller's market, with the exception of having to deal with potentially unethical types, and the seller has the upper hand there, too. I'd say to make sure he puts a lower limit on the selling price. But if he takes it to a dealer, he's not going to get more than 50% or so of the items' real values. In fact, probably less than that, as the dealer will probably offer to buy the lot at a specific price, valuing some things low or nil so he can get the prize items.

Note that I am not an expert on baseball collectibles, but do have a lot of experience with other collectibles, mostly comics and non-sport collectible cards, albeit before eBay came around.