There is no option for me to buy anything in the chat dialogue anymore, is it a bug?

Is it after shop hours?

Even if the NPC is there, if it is after their shop hours, they won't sell to you.

Nope, not after shop hours and other people could sell to me at the time.

The NPC schedule may be off. Go somewhere else, wait 73 hours, then wait again until it is the middle of business hours, then go back.

If the NPC thinks it is not time to sell, they won't sell to you. Schedules can get off kilter depending on if you fast travel to a city and when you fast travel. Especially if you bounce in and out of a town this way.

Yeah it gets buggy from time to time especially if you fast travel. I had merchants tell me to get out of their store in the middle of the day, I had their stores locked and I had them just refuse to sell me things. It usually fixes itself in a couple of game days.