What actions can I take on Ebay if buyer hasn't paid?

I'm new to Ebay and I started off really good with sales, the problem is I have many items that people have not paid for (obviously I haven't shipped their items). I have +300 dollars that I haven't received and this is about a month ago, what can I do at this point? I have already sent in invoices and I don't want to be too pushy but it is pretty irritating. -thank you