You might want to ask the mod to move this to the Hybrid side of RVNet and get more complete info. Also check Hybrid Explorer for info.
Having had an '07 ROO 23SS for going on 8 seasons, they're a great family trailer, you get a lot of room in a lighter weight and all the amenities of a standard TT. Since we suddenly realized that old age had crept up on us while we weren't looking, we recently moved to a TT.
In addition to LOOKING for problem areas, be sure to SMELL for any signs of mold, mildew, mustiness. Be sure to open up the bunkends and go over the canvas thoroughly....if it was closed up with the tent fabric damp, it will show up. HTTs are great and we loved ours, but know ahead of time that they do take a bit more set up....not terrible, but maybe 15 minutes or so more than a TT. You have to be meticulous about making sure the bunkends are totally dry when you close up or you need to open back up when you get home to let them dry completely. If by chance it's raining, open up anyway and wait it out until the rain is over and everything is DRY. If you're storing away from your home, this may be a problem for you.
Suggest you also look carefully for any little 15 minute jobs that weren't taken care of....if the previous owner didn't do those things, they likely didn't do the big ones. Upholstery stains that weren't cleaned, cabinet screws not tightened, dirty areas on walls, etc. When we sold our ROO this past year, there was no sign it had ever been used....that's what you want. It needs to look totally maintained....moreso than a TT because of the unique maintenance that needs to be done. Open up all the bunkends and the flaps....the zippers should open and close smoothly. If they aren't shown, ask if this trailer comes with bunkend fans/lights. There will be an AV jack above the bed near where the poles hook. They can be ordered if not available and the wiring is there....probably ebay and around $30 each. Also ask about Pop Up Gizmos for the bunkends. If this is a dealer, they may or may not have them, but most owners have them and they're worth their weight in gold for moderating the temps inside. They can be ordered directly from the manufacturer for around $70 each, IIRC. They are specific to the model.
Tires....check the manufacture date, never mind when they actually went into service. It will be a 4-digit number on the tire along the rim. Something like 2109 would mean that it was manufactured the 21st week of 2009. Trailer tires, regardless of how good they look, need to be replaced at the 4 - 5 year mark. A new set will be in the neighborhood of $500 give or take. If any of these things are not included in the price, take that into account in your offer.
A good WDH will be needed and also in the $500 range. Equalizer or others are highly regarded. A brake controller is required by law, so be sure you are set up for that.
We sold our '07 ROO 23SS for $9500 almost exactly a year ago. I would think you should be able to settle up at around $7000...$6500 would be better if it's missing the fans, PUGs or needs tires. Good luck and do take a look on the Hybrid Explorer for more info. HTTs are wonderful and we enjoyed every minute of time with ours. Good luck. Happy Camping!