Get a new one. u can get one with the year of PS plus for 199 dollars new. So it ends up being close to the same price and then you have your reciept and a manufacturers warranty incase it doesnt work right. It has some known issues with battery defects, and the screens are OLED and dont always display the same so its possible the screen can be defective. Its worth it to buy new just so you can be sure you have a working product that hasnt been dropped.
I agree. While some people do sell perfectly good Vitas because they are not interested in any of the games that are currently available for it, and want to spend the money on something else; people also sell Vitas that have been accidentally damaged, or failed out of warranty. You are taking a substantial risk to save only $55, minus shipping. If something is wrong with the console, you will get no warranty no matter when the Vita was originally sold. You would have to pay to get the problem fixed, and that would certainly more than offset what you saved buying used. I would advise coming up with an extra $45 (plus sales tax, if you have any where you live), and buying a new one, even if you have to wait a bit.
As for whether is is a good time; well, the price isn't going down again anytime soon. If there are games on the Vita that you want to play, then now is as good a time as there ever was.