> Often an acquisition of a company is for the set of customers.

That's a merger. You can, not having any business currently, buy yourself into one. In which case the acquisition is purely for the profits.

> I’ve just undercut what I just sold.

No you've just competed with them. If your prices are lower then you've undercut them. If their prices are artificially high then the market, a.k.a. those customers, are the ones to benefit.

> but I shouldn’t be able to compete directly with the business I just sold

Competition is _competition_. You didn't buy a market you bought an opportunity. You still have to compete against everyone else.

> I just sold using my insider information of that business.

Insider information? On a lawn care business that has no issued securities?