To sell adult physical merch the safest way is to host it on your own website and connect it to an adult friendly payment processor like CCBill and others who handle (high risk payments).
Most of these merch websites do not allow nsfw content because most payment processors such as Stripe, Paypal and Mastercard have in their terms of service that they do not allow "obscene, pornogrpahic material, including fiction and illustrations" Most people do so anyway, but at the risk of getting their account banned. It's a pain to look at the terms of service legal stuff, but it is there.
I would recommend, Printify for printing physical merch, and it has integration with website builders like: Wix, Wordpress, which has integration with woocommerce which is an open source ecommerce platform. It will be a lot of setup, but once you're set up, it'd be smooth sailing as Printify will take care of the orders for you. Printify works with individual manufacturers that have their own rules with nsfw art so you may need to check, but I've only seen that selling to Walmart does not allow nsfw merch. (Yes you can sell merch on Walmart's through printify for some reason)
I say this as someone who has been booted off many platforms that appeared adult-friendly at first only to get booted off years after being established when they decide to crack down on adult content. It is very annoying, so def try to do things by the books and don't assume.