"The Martian Shop", a novelette by Howard Fast which was also the answer to the old question Short story about a Martian invasion that starts with a mysterious set of stores in major cities; first published in The Magazine of Fantasy and Science Fiction, November 1959, available at the Internet Archive. You may have read it in one of these compilations.
It's not juse one but three shops, in New York, Paris, and Tokyo; and they offer not three but four items for sale, namely, clocks and adding machines, as well as the motors and music boxes that you remember:
The outboard motor was an object about the size of a small electric sewing machine, fabricated of some blue metal and weighing fourteen pounds, six ounces and a fraction. Two simple tension clips attached it to any boat or cart or car. It generated forty horsepower in jet propulsion, and it contained, almost microcosmically, its own atomic generator, guaranteed for one thousand continuous hours of operation. Through a muffling device, which has so far defied even theoretical solution, it produces less sound than an ordinary outboard motor. In each shop, this was explained, not as a muffling procedure, but as a matter of controlled pitch beyond the range of the human ear. Competent engineers felt that this explanation must be rejected.
In spite of the breathtaking implications of this atomic motor, it was the music box that excited the most attention and speculation. Of more or less the same dimensions as the adding machine, it was of pale yellow synthetic, the hieroglyphs pricked out in dark gray. Two slight depressions on the top of this box activated it, a slight touch of one depression to start it, a second touch on the same depression to stop it. The second depression, when touched, changed the category of the music desired. There were twenty-two categories of music available—symphonic music in three chronological sections, chamber music in three sections, piano solo, violin solo with and without accompaniment, folk music for seven cultures, operatic in three sections, orchestra, full cast and orchestra, that is the complete opera, and selected renderings, religious music, divided into five religious categories, popular songs in national sections, instrumental music in terms of eighty-two instruments, jazz in five categories and three categories of children's music.
The salespeople in each of the three shops claimed that the music box had a repertoire of eleven thousand and some odd separate musical selections, but this, of course, could not be put to the test, and varying opinions on this score have been expressed.
