Dealers often use WBAC to value trade-ins. WBAC is owned by British Car Auctions so it could be fairly accurate valuation of what the car will fetch at auction.
Your point is true to a degree.
However, the valuation from WBAC needs to be lower than the auction price. They have costs in their operation even if they are just aggregating volume for BCA without profit. If WBAC are covering their costs/ making a profit, a dealer valuing at the WBAC price will also make WBAC costs/ profit on the auction sale of the part exchange as well as the sale of the new car. The dealers marginal costs will be much less than WBAC full costs so it is very attractive to the dealer to quote a WBAC valuation. (That's why they are called dealers!)
So as a rule of thumb, a WBAC valuation is likely to be lower than the market rate. How much lower I don't know, but I suspect a fair amount?
Like all things it is best to do your research and get at least three quotes each for buying the car and selling the car. The least "cost of change" option will likely be a private sale and purchase through a car broker. Some don't like selling privately (me too!), some don't like buying though a broker - so you pays your money and takes your choice. I found when I was in the market for my Jazz, I had some very, very, silly (naïve) "cost of change offers" from Honda dealers.
It is interesting to take the car to a local car dealer who buys cars for cash to see what they offer.
That's what I did - and used a broker, who found a Honda dealer quoting about £3,500 off an unregistered brand new Jazz EX CVT with all the normal Honda benefits included just as a normal customer. (I bought the car directly from the dealer and paid the dealer in exactly the normal way except the price and with no haggling).