After Glynetha Twill told you that the songsheets will be random , the only option you can take is "Nevermind". So you are not able to buy any songsheets from her.
Is it possible you didn't have enough gold? You wouldn't buy be able to buy it if you lacked the 400 gp needed.
I don't see any issues with the conversation...
I had about 1200 gp at that point. Will retry later.
Retried it with more than enough gold. Still the same: ,,Nevermind." is the only given option.
Ah, I see the issue. She only sells to people who are members of Ticker Square. I will make the conversation more clear.