Unlike many other forums where there are encyclopaedic lists of rules, CS has only a few simple rules which we ask that members observe.
WTB posts require a price as this is not a reverse auction site.
As Mal suggested in your post which was edited, if you are prepared to spend a few short minutes browsing through the WTB pages, youll get a good idea of whats a fair 2nd hand price for the machine you were interested in. You can then post and if your prepared to pay price is fair and if there is a member wanting to sell your required machinery, you can negotiate final price from there...
Bottom line is that it will save all parties from entering into a fruitless negotiation with unrealistic expectations on price. Hopefully thats not too onerous? *:-?