If a uke does not come with strap buttons, I add them myself, even my most expensive custom ukes. I also feel they make the uke more valuable. I never buy a uke for investment, or even think about what I would get out of it. When the time comes to let it go, I let it go.
Expensive or inexpensive ukulele, I put them on the ones I own because I fully intend play them. In fact, a uke looks more "normal" to me with them on than off. Keep in mind that I do play almost exclusively tenor. The one concert that I currently own still has a button that came stock on the bottom.
My thought about buttons and resale value is that the vast majority of second hand instruments sell for less than the same ones brand new, regardless of buttons being present or not. So add buttons if you think you'll want to play standing, and don't if not?