The name of a mythical super-collector, who always gets to flea markets/thrift stores/yard sales/boot sales/etc. and purchases all of the worthwhile merchandise you are personally seeking out. Also spelled as "Sum Guy."
Started in rec.games.video.classic, as the name of an individual always invoked by sellers who would tell inquiring collectors "yeah, we had a lot of <insert collectible here> until some guy came in and bought them all." That line, or variations, was so common, that it was determined that there was a single person wreaking all of this havok, and the sellers were actually stating his name- Sumguy.
Applicable to all frustrated collectors who shop in vain for their favored items.
I found a Vectrex overlay at this yard sale; I asked the guy running it if he had the Vectrex still, and he said that he had the system, a few dozen games, the 3D Imager, and light pen, but that Sumguy had just bought it all less than 5 minutes before I got there!