Mentally unbalanced individuals selling weird things on eBay always seem to use a minimum of 36-point type. Kind of like the TimeCube guy. People selling normal things, like china or slide rules, don't do that. It's a dead giveaway.
And of course the wackos also have the tendency to use multiple exclamation marks. Terry Pratchett, among others, has said that it's the sign of a diseased mind. Thinking back on some people that I knew personally who were in the habit of doing so, he might be right.