You need to be an Established Member first before you're able to post in the 'Buy, Sell or Trade' forums. Check this link for further help - https://www.digitalpoint.com/help/established. You need to wait till you have 3 'likes' from 3 different Established Member. Alternatively, you can also purchase the Premium Membership so can you can immediately access and use the BuySellTrade without having to wait for the 'likes'. Also note that even if you become an Established Member, you cannot use some specific areas of BuySellTrade without the Premium Membership. Hope this helps.
No worries mate. Just make sure you post only useful and informative stuff that others find helpful. The 'likes' will come in no time. It's not at all hard to get those. Just be patient and keep on trying. Cheers.