I usually have success by throwing out a low but reasonable offer, but at the same time backing it up with justification for the price, be friendly and create a dialogue with them, don't just reply in an email "$350?".
Say that you like the set, but a lot of tools look like they have seen a lot of use, you really are only interested in the cabinet and sockets because you need them to earn some extra money working on cars on the weekend and you already have plenty of pliers and hammers already, and this looks like a nice starter set you can use until you can afford to buy some "new" tools. Let em know that you are very serious, have cash in hand and can come at their earliest convenience.
You may ask if their price is flexible, but I wouldn't make an offer til your there, you can him and haw and find little defects to justfy your rationale.
If you want them, I wouldn't hesitate, someone else will snag them up.
Good luck!