Hi! Thanks a lot for your help my account has been upgraded I see that you are on this platform since 2015 can you please give me some tips about optimization and start selling. Thanks again for all your help.
Actually I've been a member here since 2002

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To find the "Categories" button look in the dark blue stripe across the top of the screen. It's second from the left.
As far as how orders are placed and payment made, you start a thread in whichever section of The Exchange is most appropriate for your membership level and what you're selling, you post a description of the knife, preferably several good photos, post a price that includes shipping costs, indicate which manner of payment you want to use (PayPal, money order, etc), then if another member posts that they want to buy, or sends you a private message indicating they want to buy, you arrange payment and shipping. It is customary for payment to be sent to the seller before the knife is shipped to the buyer.
To find The Exchange (Knife Exchange), it's the button in the blue stripe at the far left at the top of the screen
Bladeforums does not operate like Ebay. Bladeforums does not provide a format for people to place orders or collect payment for the seller. The Bladeforums Exchange operates like classified ads, you have to do everything yourself. And there are no auctions permitted here like on Ebay.
And you will most certainly want to read the rules of The Exchange that sbh06 linked above. Read them, learn them, live them. If your sales post violates those rules it can be deleted.
If you are looking to advertise yourself as a custom knife maker in the hopes that people will contact you to ask for knives to be made, start a thread in The Custom Shop: Services Offered subforum.
Also, with a Knifemaker/Craftsman/Service Provider membership you can request your own subforum here at Bladeforums.