ThredUp. I thought the fabric and styling were really cute (and the price was right, and it doesn't have the environmental or human impact of a new piece of clothing!) but it was too big.
Here are the before pictures, front and back. It was really baggy and the neckline was shifting and gaping all over the place. I thought about taking in the sides, but it's a woven (non-stretch) pullover style, so I was worried about still being able to get it on and off, and it has pockets and a bottom hem band that I didn't want to mess with. I would have probably also needed to take in the shoulders to raise the neckline, and I was afraid to mess with the tulip sleeves.
Instead I decided to take the easy way out and put in an elastic casing in the back. This idea was inspired by the Washi dress and Josephine top by Made by Rae. I pinned in a makeshift casing on the outside to audition elastic length and location - you can see that in the top photo. Then I cut a piece of white fabric to 1.25" by the width of the shirt back plus one inch (21" in this case), pressed the edges under, and stitched it to the inside of the shirt back, just under the armscyes. Here is the after. It is still kind of baggy - there's no way to change the style of the shirt, and that's okay with me. But I no longer feel like I'm swimming in it, and the neckline stays in one place now, close to my chest. Robert says it even almost looks like it's supposed to be this way, so I'm counting it as a success!
I'm not counting this as a start or a finish in my running counts.