There is nothing wrong with using old bars. I've done a lot of cars and never bought any new bars.

They are marked left and right though. If you crawl under the car and look at the bar end, you may have put the L or R in towards the back. If you can see just one is in its correct location you are good.

If you have the bars in the wrong side, hummmm. I would think you may have a future problem. If they have been in there wrong for a bunch of miles I'm not sure I would trust them if you put them back in the correct location.