need help with tomestone exchange please!

Hi everyone, I'm noob so please excuse my ignorance but does anyone know why I would not be able to exchange irregular tombstones or lore? I am at the vendor and I have enough for what I want ( a mount) but when prompted to trade the trade button is greyed out! I have googled and I can't find anything.....apparently I should be able to exchange even when the event is not on, I just can't get more of the tomes?

In the prompt, you should see a greyed out image of the tomestone. Select it and then select the tomestones in your inventory, then you should be able to confirm the purchase.

OMG Thank you soooo much lol I'm such a noob!

It's a bit confusing because they don't function as you usually expect tomestones to work. The normal ones are a currency, but the irregular ones are items for bartering.