I’ve completed the main line quest past the point where Grace directs you to open a storage shed and the trading post, played for over 3 hours, and still nothing. I started on a free weekend, but purchased the game over a month ago.
Sorry to hear the game still don’t let purchase or sell gear/items at the trading post, the pictures are prove enough that you meet the requirements.
Some hours have pass since you post your issue, is still affecting you?
If it is, I encourage you to re-start your PC and Steam again; then contact AGS customer support to rise a ticket if the game is still preventing you to trade.
Thank you for your understanding and patience. Wish you the best and a nice week.