Low balling customers

I recently sold a small trailer to WeBuyCars. The first offer I received was R4 000, but when I went back a few weeks later, they offered me R3 000. The reason given for the drop in price was that the trailer (a 1994 model) would decrease in value in the following year. They refused to pay any more. I have since seen the same trailer being advertised for R9 000. I understand that everyone needs to make a profit, but making a 200% profit and coming up with long stories just to justify dropping the price is not on. It may be a case of sour g****s, but I don’t think what they are doing is right. I won’t be using them again.