Kls-- Since people are questioning the credibility of your post, why don't you post a picture with your username and the date on a piece of paper next to the inverter. This is a common way to at least establish you have the thing that you are selling.
Since you are very new here, and the item you are selling is listed as sold on some other website, its natural people will be skeptical/cautious. Posting the additional picture may go a ways towards clearing that up, if this is a legitimate sale.
Everyone Else-- exercise caution and do your due diligence when buying things through this forum or any other. Most people are trustworthy, some are not, and some fall in between.