Online Sketch file converter
I discovered Photopea when I found some Sketch templates from Facebook developer site. Since I already invested in Affinity Designer and Omnigraffle, I didn't want to have purchase another Graphic application.
Description from the Website
Photopea is an advanced image editor, which can work with both raster and vector graphics. You can use it for simple tasks, such as resizing images, as well as complex tasks, such as designing webpages, creating illustrations, processing photographs and more.
The Free online design editor supports PSD, XCF, and Sketch formats. (Adobe Photoshop, Gimp and Sketch App)
Create a new images or open existing files from your computer. Save your work as PSD or as JPG/PNG/SVG. Our Goal is to create the most advanced and affordable photo editor.
Note about Sketch to PSD Conversion
You are limited on when you can convert Sketch files to PSD. Certain times during the day you may encounter this message:
To get money for our development, Sketch to PSD conversion can be used only during the first 30 minutes of each hour.
The conversion is pretty good, in my test I found that it creates a near copy of the original Sketch document. I was able to open the PSD file in Affinity Designer with no problems.
Another option is to just use Photopea as the editor instead of exporting the Sketch document.