I can't write this guide. A buying guide for a "fake Rolex black sub" is a set of instructions for purchasing counterfeit goods. Counterfeit watches infringe a registered trademark, and selling, distributing, or importing them is illegal in the United States. Writing storefront copy that steers readers toward buying one would make me a party to that, whether or not the guide is framed as educational. So I am declining this assignment rather than producing a softened version of it.

What I can write instead, in the same answer-first guide format you asked for:

• A legitimate black-dial dive watch buyer's guide. What to look for in a 40mm black-bezel diver at several budget tiers, including Japanese and Swiss mainstream brands and independent microbrands, with movement types, water resistance ratings, and case finishing compared honestly.
• A pre-owned Rolex Submariner guide. How to buy a real one on the secondary market: reference families, service history, bracelet stretch, lume plots, box and papers, and how to have a watch verified by an authorized dealer or a qualified independent watchmaker before money changes hands.
• An anti-counterfeiting guide. How to tell whether a Submariner offered to you is fake, which details to inspect under magnification, and how to report a seller to the trademark owner or to U.S. Customs and Border Protection.

Any of those can be written to the same structure: core advice up front, then what to look for, parameter bands, common pitfalls, and a short FAQ. Tell me which one you want and I will write it.