The Sora component line from Shimano is garbage, I wouldn't recommend that bike. In a nutshell, the Shimano lines for road are: Sora, Tiagra, 105, Ultegra, and Dura-Ace. Tiagra is the lowest a regular rider should go. There are different brands for their mountain and hybrid components, BTW, the ones to avoid there are Tourney and Altus.
The Trek 7300 FX is a MUCH better bike, I'm quite familiar with them. The seller is definitely inexperienced at selling bikes based on the photo angles he chose, so it's very likely this is underpriced. From what I can tell from the photos it's in great shape, all components look extremely clean and in good condition (from what little I can tell with the crappy angles).
I don't know much about the Specialized but it's probably decent.
The first listing is total bike porn: that's the bike equivalent of a 1960s Mustang. Lugged frame is classic, they don't make 'em like that anymore, and it looks like it's in excellent condition. If you were trying to commute 10 miles I wouldn't recommend it because the components are dated, but it could be upgraded with newer style components and it would be one sweet bike worth $500-600. If you took that bike into a shop the mechanics would be drooling.
The only possible downside to the Motobecane is that it uses caliper brakes, which may not be compatible with fatter tires. The way you tell is by actually opening up the brakes and noticing how far apart they separate. If they don't open enough, you wouldn't be able to stick a wheel through them if the tire were too fat. But I don't think this should stop you from at least looking at the bike. The seller looks like he's pretty experienced at selling, judging from the photos and description, so he/she should be able to tell you (it's even possible to demonstrate if it will work if the seller has a x32 or x35 tire lying around).
