When you see multiple listings from the same people, repeated day after day after day, they are making a good part of their living from flipping bikes (or trying to) and to me that crosses the line from what a hobbyist does to 'thin the herd' or even to fund the bike cache. I don't even think that the casual bike buyer will care; they'll look under 'both' and maybe even prefer buying from someone who sells lots of bikes.

The distinction may actually be of more interest to the IRS, as at some point they are going to have to figure out how to get their pound of flesh from those who sell a lot of stuff but don't report it as income. The proverbial once-a-year-yard-sale they can (and do) overlook, but people who essentially have a yard sale going every day are a different matter.