Good points. It is possible that the buyer does not have enough tokens to improve his bid if he already made a first negotiation bid.
There is definitely a sort of bug on the seller's side in negotiations though (at least when using the browser version 2016). This season someone sent me a negotiation bid for one of my players. I set an asking price, but then I decided to change it which could not be done; the seemed to be a sort of malfunction in the 'drag' and then an error message poped up, and the game reloaded by itself afterwards. I tried to change the asking price several times, and after each try, the same error happened and the game reloaded.