Is the 有 in 你有想要买什么吗 some kind of auxiliary verb?
你有想要买什么吗. It sounds more like Taiwanese usage of 有 to my ear. I found they often put 有 between subject and verb. E.g. 我有去过；我有看过；where I often just say 我去过；我看过. In this case, I'll probably say  你想要买什么? or 你想要买什么(东西)吗?.