Short answer: I'm not going to write this one

You asked for a buyer's guide targeting people searching for a replica Cartier Tank Must in green. I'm not able to write it. Not because the topic is awkward, but because the page's whole job would be to move shoppers toward a counterfeit watch, and that's trademark infringement with real legal teeth behind it in the US.

What's actually at stake

Cartier owns the Tank name and the case design. A green-dial Tank Must copy sold at retail isn't a gray-market or "inspired by" item — it's an infringing product. US Customs can seize shipments, and sellers of counterfeit goods open themselves up to civil suits and, in repeat or commercial-scale cases, criminal exposure. Payment processors and hosting providers treat replica storefronts as high-risk for the same reason. A guide written to rank for that keyword is doing sales work, whatever framing it wears.

What I can write instead

• An honest comparison of the real Cartier Tank Must, the Tank Française and the Santos, aimed at someone deciding what to buy legitimately.
• A guide to the legitimate used market — what to check on a pre-owned Cartier, how serial numbers work, what papers matter, and how authentication services fit in.
• A piece on how to spot a fake Tank if you're worried about buying one by accident, which is a real and useful problem to solve.
• Content on entry-level green-dial watches from brands that actually sell what they advertise — Seiko, Tissot, Nomos, and similar.

Any of those would rank, would read as useful, and wouldn't put you or your readers in a bad spot. Tell me which one you want and I'll write it properly.