Tax should be deducted from the amount you put to the trade, so the receiver gets less than the absolute value.
Reason why that trade fails might be to the clintz update lag- something that happens on UR every now and then. At those cases what you see on your account is not what you in reality have, because your latest buys or sells have not yet been registered fully. this is longstanding technical issue that fixes itself in few minutes upto about 8 hours, depending on unknown reasons and there is nothing you can do about it (refreshing browser caches does not help etc.).
If you have enough money on account (in reality, according to server - not according to your user interface) - you can make transaction.
Way to know if you have this update lag issue is to sell some cheap card and go look main page, after you get the sales done the sales message displays your real balance in it like this -> "jepepe has bought your Dolly for 279 Clintz (32 159 Clintz in the Bank)" - if that "in the bank" amount is different from what you see on left panel you are subject to lag. The message from sales displays right amount of clintz (since it comes straight from the market accounting subsystem instead of lagging part of the player profile).
this might be the reason...