If you do go with a used machine make sure you check it over well for damage. This means are there cracks? Is there a haze on the screen? Are the ports in good shape? You should also be cognizant about the keyboard, does it work? Are there missing keys? How does the trackpad work? And of course does this used machine come with any software that works or are you on your own? Will it need upgrading the minute you try to turn it on?
Basically, know what you are buying. Do your homework. Look up what comparable models are selling for on online stores and resale sites. Look to see what original owners liked and disliked about a certain model and when all else fails, look for a new one within your budget.