I wanted to buy a wireless headphone and I have a couple of questions to ask about it.
On which site should I consider posting my question?
None. Shopping questions are off-topic throughout the Stack Exchange network.
See this blog article for a more thorough discussion of the rationale: Q&A is Hard, Let’s Go Shopping!
Do note that the Stack Exchange system powers Q&A sites, not "forums". That's probably the source of your confusion. Questions here need to be specific, answerable, and useful to other people. If you can imagine people having a long discussion about your question, or that there are multiple correct answers to your question, then it's probably not a good fit around here. All of this is discussed in the FAQ for each site; for example, the Stack Overflow FAQ. I do recommend that you read it.
If you really want to associate buying and Stack Exchange, check out the Stack Exchange store, where you can buy cool swag!