If you are selling art beware of scammers. As a rule of thumb never click an external link that you don’t recognise.
If you are selling NFTs a scammer will typically offer you a “too good to be true deal” and then give you a link to an external site. I am listed on objkt.com and all selling and buying activities can be done within that site.
And -pay for promotion- offers, I have never paid for one but I have used Instagram Boost which has been effective and surprisingly cheap.