Yes you can
I believe you can bring your own merchandise in, but I have found that not all the musicians are available to sign.
You can bring something, as far as I know (I've never run into issues with it); artists usually have merch to purchase as well.
It would probably depend upon the artist, but there are no policies against it per se as far as I know. Artists typically do have their own products such as CDs and t-shirts to sell, of course.
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