Sizzlestix

Hi, I'm new to cycling and I recently purchased a Trek 4300 Disk mountain bike from my LBS. I planned on doing some offroad trail riding, but I've ridden about 30 miles so far and they've all been on pavement. I think I want to stay on pavement for now, but the shop won't do any trade/exchange deals and I haven't got any hits on my Craigslist ad, so I think I'm just going to keep it and try to do some trail riding in the future.
However, I'm now looking to get a road bike as a second bike. I want something that is a decent entry level bike, but I don't want to spend a lot of additional money. I've been looking at used stuff on Craigslist and I came across a 2007 Iron Horse Triumph 3 which has been upgraded to Shimano 105 components. It is listed as having slight cosmetic damage on the frame and shifters. The ad is listed for $600, but I made an offer for $300. He countered with $450 as long as he could swap out the 3T Doric Team seat post with an aluminum one and put his older Fulcrum Racing 7s on the bike. The Fulcrum wheels are described as "a small dent in the rear rim (can't feel it riding, but there is a small bump when applying the rear brake." He then agreed to include some Keo Classic pedals into the deal.
I'm 5'11 and this is a 56 cm bike. My MTB is 18 inches and fits well. Based on what I've read a 56 cm road bike should fit me. I would definitely do a test ride before purchasing, but I've never been on a road bike and I don't know what feels right or wrong. Can anyone give me some advice on this and let me know if this is a deal worth pursuing or if another counter offer would be appropriate? The link to the ad is below. Thanks.

https://lexington.craigslist.org/bik/3320667362.html

Retro Grouch 

Buy a set of smooth tires, like maybe some Schwalbe Big Apples for your mountain bike. That shouldn't break the bank and it'll completely change the character of your bike for pavement riding. Switch back to your knobby tires for off road rides. After you ride a few hundred miles you'll be a lot smarter regarding exactly what kind of bike you need.

Sizzlestix

I think I'll try that first...my bike has 26' wheels, will they fit? Anything else I can do to make it respond better on pavement?

Mondo734

If your bike's front suspension fork has a lockout I would make sure that the fork is locked. This way when your pedaling your not bouncing around and more of your effort goes directly to the cranks.