I'm not sure if it was this forum or another, but recently a person bought a used bicycle and when checking the bottom bracket found out the frame was damaged and needed either repairing or replacement which was probably in the neighborhood of $300-500. Having no warranty and buying a potentially seriously damaged bicycle is a downside to buying used.

On the other hand, if everything turns out fine, buying used can give you a lot of bang for your buck. The last three road bikes and frames I bought were all used.

If you're new to road cycling though, there is nothing wrong with buying an entry level bike ($700-800) new at a shop. They are quite good nowadays, the major difference from more expensive bikes will be that they tend to be heavier with a few less gears. If your budget is $1500, that will buy a very nice new bicycle which is not entry level, and you could potentially get a bicycle that retailed for $2000 or more when buying a clearance model from last year or older.