It's good. Should speed up entry and improve efficiency. They are merely leveraging technology, anyone who struggles with this shouldn't be travelling.
Why would it be illegal? You could sell the whole bike or parts, makes no difference. Selling an item is neither "work" nor a "business". Do you really think Thai authorities are chasing after people on Facebook marketplace?
I did exactly that a couple years ago. My visa didn't come through and I had to enter exempt. Got to front of line at 11:55pm so I let the guy behind me go ahead of me. The extra day saved me from an overstay.
I'm assuming you have a Thai GF? If not, why do you want to settle in one place? Thailand is decent but 5 months is enough for me. If I were in the same situation, I'd use Thailand as a base, but would spend as much time elsewhere as in Thailand.