I really need to start going to bed sooner... but until then I'll surf the web...

OP, most of those bikes seem way overpriced. I would most certainly stay away from bikes with shocks in the back (called soft tail I think). The one you posted was the diamondback. Those are designed to be ridden on very demanding off road courses. I had a cheap target one as my commuter (until it was stolen, thankfully) and I hated it. The extra suspension causes you to "bounce" a bit while riding, which is very uncomfortable on the road and very inefficient. So I'm pretty sure everyone here will agree soft tail is not for the road.

Second I am always wary of someone who says the bike has only been ridden n times. Its especially hard to believe when the bike is dirty. I'm not saying that they're lying, but I wouldn't be inclined to believe them. Plus the bikes that have only been ridden a few times or miles that you posted are several years old. Older bikes may have not undergone the maintenance necessary to keep it in good condition, or some parts may need to be replaced (e.g. tires, chain, brake pads) depending on the conditions it was stored in. Long way of saying they are overpriced.

I've seen this up close in person, actually helped assemble it. The directions are "fairly" helpful. You'll just need a multitool (I just gave you an example, any basic multitool or allen wrench set will do) to assemble it.
Its actually not bad at all. Fits into your price rance, its fairly adjustable, and suits your type of riding. Instead of the "knobby" tires that mtn bikes have, it will have a smooth-ish tread, which allows for a better roll on pavement. Don't worry, smooth tread tires will provide all the traction that you will need on pavement; the knobby tires are for gripping things like grass, mud, and gravel. Plus the bike will not have any shocks (aside from a very one on the seat post - not a big deal), which keep the price down and the weight of the bike down. Imo, shocks are completely unnecessary for pavement riding; they are only needed for fairly uneven terrain. The bike isn't up to the "caliber" of bikes that most people ride here, and its still a bit below the Giant I showed you earlier, but it works just great. If you check out the size chart and order one that fits, you'll be happy.

p.s. to all you grammer people, I checked for content, not not grammer. deal.