Could be one of several things:
(1) Stolen vehicles
(2) Some sort of advance payment scam: "I'm broke and got the bike towed. I need a check for $250 to get the bike out of impound... I'm only selling it cos I lost my job..."
(3) A way to collect e-mail addys or working phone #s to sell to other scammers or marketers
(4) Something worse, like people are encouraged to bring cash when viewing the bike and get mugged.

Either contact the police, or contact the person placing the ads (anonymously) and attempt to feel out the situation. If they're selling stolen bikes and you can get them back, you may make some fellow riders very grateful. Bike thieves are the scum of the earth.