Not sure I understand the "Buy Box Eligibility". I frequently have the Buy Box on LEGO listings with my personal account. I have seen comments like this on Seller forums, and I thought it meant you get Buy Box on listings for your own products (i.e. where there is no competition because the products are items you make).
UPDATE: Just pulled the trigger on moving to a Professional plan. After confirming my Credit Card details, picking the "Toys & Games" category, and giving an indication of my listing volume, I had to do a txt-message verification and was immediately approved. Account shows Professional and all my listings are still active. I now have access to Tax Settings as well (no settings there when you are on an Individual plan).