This happened to me once before. Go to your steam folder then in the steam files delete everything except for the steam.exe, Userdata folder, and your SteamApps folder. Then restart steam and let it reacquire the files you deleted and then there should be no problem making a purchase.
Good luck!
Edit: This is what Steam Support tells you to do.
If you're still having trepidation in doing these steps, check this video out.
He already said he tried the purchase via his phone which sort of implies it's not just related to an issue on his PC.