So I am reading about Mining of invalid blocks since July 4 and how this allows for double spending. I don't understand full technical details of what this means but a weird situation is happening on my favorite bitcoin trading site. Lots of people are accepting paypal payments very close to going exchange rate. Usually when accepting paypal, the seller asks for a 30% premium due to risk of people rescinding paypal payments. Also the listings show specific amounts. What appears to be happening is these sellers are selling the same blocks of bitcoin to multiple people. So look out for people selling specific exact amounts. In the above list IvanRussia's selling advertisement and the one above appear to be fishy.