Firstly, thanks for an honest engagement.
In no way or place does the RAW state that items with Availability must be purchased on the black market.
P419
"BLACK MARKET GOODS
When you get to the good stuff, the higher the Availability Rating is, the harder it is to acquire the item. To purchase an item
off the books, make an Availability Test."
The section going on to detail the mechanics of making these purchases.
The use of the term "off the books" means the black market. The title of the section is 'Black Market Goods'. Applying these rules to retail purchases would be a (perfectly reasonable) houserule, but a houserule nevertheless.