I am having this exact problem so you are not alone. My friends can buy and sell things but I can’t buy or sell things. I am yet to buy a single thing on the trading post because the buy button isn’t working for me.
I thought I was just a idiot and doing something wrong but I am pretty sure this is a glitch or something guild wars is doing for a reason. Either way I sent a suppourt report showing and would love to know if guildwars is addressing this or is doing it for economic reasons.