Agree with Laural, sometimes a used package deal can be better depending on what all it comes with. In my area you can get full setups with lights, filters and stands usually with the aquarium at 1/4 the price of buying it all new, often times the lights aren't what you are going to want for a planted aquarium but it may get you started. When buying used aquariums, or the cheap $1-$2 a gallon aquariums, there are a few things you need to look for:
Your main focus needs to me on the silicon seals, make sure they look good, there aren't any gashes in them, no chunks missing, and when you press it with your finger it should be squishy not hard as a rock. Another test I do is where the glass meets in the corners on the outside edge, I will poke it with my thumb nail, it is feels squishy and does not leave a dent that is a good sign.
Second focus should be on the glass, check all the corners for chips, missing chunks, and cracks, then check the entire glass surface, even the bottom! Give the glass a gentle push, it shouldn't bend but should give slightly (depending on thickness). Look at the front and back and check for any signs of bowing, this is a bad sign and any bowing is not acceptable to me. Take time and look at the plastic trim of the aquarium, look for cracks and missing pieces, if the aquarium has a center brace take time to look at it, grab it and bend it slightly back and forth and check for repairs, this is the most common thing to break on the trim.
If all of that looks good, if you are still unsure ask to fill it up, most people will let you fill it up, if they don't want to fill it up for you and you aren't 100% sure on it feel free to post pictures here or just pass on it, there are always nicer people out there. It is also good to take these steps when buying aquariums from the store, often times they are quickly made and mistakes happen.