In my home office, I have three large IKEA bookshelves. I acquired them separately over the years, but made sure to always buy the same exact model and finish so that, now that they are in a row on the same wall, they look like a beautiful, built-in shelf, where all of my books are finally in one place after many years as a student. But they are not built-in bookshelves; they are only attached at the back with hooks so that they don’t fall over.
When you are buying or selling a home, it is extremely important to be clear about what stays and what goes. If you have something that could be considered a fixture, i.e. that looks like it is attached, you should put it in writing that it moves with you. Conversely, if you want something to stay, you should make sure that you specifically include it. Calling after the fact to say that the microwave looked like it was built in won’t get you very far if it was only sitting on a shelf.
I know that when I sell, even though they are chattels, I will make sure that my bookshelves are excluded fixtures.