The sitting president is selling watches

From the site

    Trump Watches are not designed, manufactured, distributed or sold by Donald J. Trump, The Trump Organization or any of their respective affiliates or principals. TheBestWatchesonEarth LLC uses the “Trump” name, image and likeness under a paid license agreement which may be terminated or revoked according to its terms.

Trump doesn't sell them directly. None of the stuff with his name on it comes directly from him. It's all licensing deals or attached to a PAC/SuperPac/Political thing.

Campaign web sites often sell merch. But the money belongs to the campaign, not the individual. And it's treated as a donation by the FEC, so you can't use it to get around campaign finance limits.

I recall that there was a lot of complaining that Obama made a fair bit of money on book sales while in office.