Was working fine but all of a sudden trade offer url does not work. I havent changed it at all I tried to check if it was right and the one on the system is the correct trade offer url as I can use it to receive trades. I can just imagine this is a bug. Every time I try to buy keys for bitcoin it goes trade offer url is invalid.

Maybe temporary? Any Steam lag/down may report this message as well. Could you try again and see if the problem persists?