I have been ridiculously lucky when buying hardware, but I am told it is not that difficult, even today, to run into machines with hardware that is not well-supported by Linux.
So I think I would actually prefer if vendors sold machines that have been tested to work well with Linux, kind of like the "Designed for Windows"-stickers on PCs and notebooks.
If a machine is made of hardware that Linux supports well, I do not care what OS is preinstalled, since I will probably wipe it and install one myself anyway. But having a guarantee from the vendor that I can do so without trouble or fear of bricking my machine would be helpful.