I'm selling my trek 4900 mountain bike-I don't know any info.about bike or even at what price to sell at? HELP?

Bicycles, like cars, depreciate in value quickly. Bikepedia shows the bike went for $619.99. My honest opinion is you'll be lucky to get $300. People normally don't want to shell out big bucks for a used bike - no matter how good the condition is. If you're in no big rush, advertise it at $300 firm and don't back down. Sooner or later, someone will buy it.