I USED TO WORK THERE. IT IS A SCAM. YOu will get your car sold quicker on ebay or in the auto trader. Nobody hardly can find onestopmotors.com in any search engine under any meaningful search phrase. The have people working on a bunkload of crap back door domains to keep the complaints against them from showing up on the internet. They joined Rip Off Reports BS program that is more or less a bribe to give them positive ratings.
JUst use the word "scam" or "rip off" in your internet searches for them when checking them up. Ignore all the bogus domain names they maintain to keep you from finding the truth on them. If they were a serious legit company they would not have to do this. Does ebay or auto trader have a dept that just concentrates on making sites to dilute out the complaints on the internet? No. In fact you can complain on their site in their forum if you want to.