eBay – How to View Old Purchases

That's not how it goes. Any user can post a higher bid than your current one, there's only a minimun (the bid increment). If your maximum bid is still higher than the new bid, ebay will add the bid increment to the last bid and count it as your new bid, so you're still going to be the highest bidder, as long as no-one surpasses your maximum bid.

Now to why you shouldn't enter 99,999$ as your maximum bid... If your maximum bid is ridiculously high, you'll be sure to win the item, but you may end up paying a lot more than the original value of the product.
In your example, as long as someone is interested enough in the T-shirt to bid up to $100 for it, if your maximum bet was 99,999$ you'd find yourself paying $100.25 for a T-shirt that in other circumstances you'd only have paid $15 for.

I believe some 3rd party eBay "sniper" software (which sometimes run as a web service, rather than desktop software) will do this sort of thing for you. The sniper software is generally used to automate the process of placing a bid in the last few seconds of an auction to increase the chances of a win.

From techradar's review of "The best free and paid eBay sniper software" it states that for "Auction Stealer" and "Auction Sniper":

...you can also place your bids in groups, so you can make bids on multiple auctions of the same type of item. When you win one, the others are cancelled.