Years ago I found a Chief RPA mount for $10.00 on ebay with $1 shipping. It was an open box but it looked unused and has served me well. So I'm a believer that it's possible to find deals like this once in awhile.
As to this listing, the seller has decent ratings. Try contacting them and ask how they came across so many. Looking at the rest of what they sell, my guess is this was the leftovers from a storage unit, business that went under, etc. and that they will buy and sell just about anything if they can make a profit on it. They probably move enough merchandise that they just mark it up a little and call it a day and don't bother to research the value of items they are not familiar with.
Are you in the UK? If not you'll have to check and make sure they will deliver to where you are located.