Believe it or not, when it comes to indiscriminately amassing as many cards as you can of one player, ebay is not the best option. In fact, it's more of an option of last resort. Large lots of cards at reasonable prices are the exception and not the rule when it comes to ebay.
That's not to say you shouldn't regularly check ebay. I monitor it almost compulsively for new Wallach items. How do you know if your checking it enough? Well, a good way to tell is if you've started sorting search results by "newly listed" instead of "ending soonest." This way you get first crack at everything, and no "Buy It Now" is going to get snagged by someone else first. If your at the point where you find yourself checking the ebay app on your phone while stopped at a traffic light, you should probably dial it back. Wait until you get home. (or so I've been told)
Mostly what I find on ebay are odd ball Wallach items. But when a good deal like the one above does show up, I'm still very happy to place a bid. This lot was for 71 cards, and came in well under the 10¢ a card ceiling that I loosely enforce upon myself.
Check out this sellers business card. It's included with the cards above, and I put another picture below. He also sells marbles. I didn't realize marbles were collectible. It makes sense when I think about it, but I guess I had never thought about it. I'm in no position to judge the hobby/collecting habits of anyone, so I don't point this out in a disparaging manner. I'm just pointing it out because I find it interesting. I'll have to keep an eye out for MLB licensed marbles on ebay.