I'm am looking to sell completed models but do not want to disassemble them. Can someone point me in right direction?
Welcome. You can sell these easily on EBay, just mention in the description that the sets will be shipped assembled. Two things to keep in mind:
Since most shipping costs are calculated by box dimensions, your shipping costs will likely be higher than when you ship the sets broken down.
Please make sure these sets are NOT glued ... you do not have to mention this in your listing, but absolutely should if they are (even if they are only partially glued).
I have personally bought both assembled used sets as well as disassembled ones, and have no preference. I always disassemble and wash before I reassemble. The only issue I faced was when I bought a set and it turned out to be partially glued, which was not mentioned in the listing. That was an easy SNAD case (Significantly Not As Described) and a full refund from the seller.
I would recommend selling on Bricklink.com. You can create an account, start a store, and begin selling without too much hassle. There are some great guides on how to sell sets on Bricklink, including this one.
As far as the actual selling goes, I would simply make sure to describe the set to the best of your ability, as Phil B. mentioned. Is the set missing pieces? Have the colors begun yellowing? Is it glued? Every buyer appreciates a clear description of what they're purchasing.
In my experience the sell on ebay brings a much faster result but with a realistic chance that your price goal is not met. On Bricklink you may have to wait for someone who is willing to pay the price you are asking for, which can take months or will never happen. With both platforms the price will be reduced significantly when you can not deliver the building instruction, it wil be raised when you also can offer the box. Btw, really a nice collection ;-)