See if you like it first. The fit. The feel of the controls. The color. The style. Then check out condition. A 7300 that was stored in doors and used only lightly, and well maintained, is worth a lot more to you than one that was rode hard and put away wet, so to speak.
Second, you could consider what a quality new bicycle would cost you. Something like the FX 7.2 would cost you around $550. A 7.3, about $650. On the other hand, an entry level Giant Escape 3 goes for $330, the next step up Escape 2, for a little over $400. So, figure between $330 and $600 for a quality new bicycle of comparable or better quality to the 7300.
Bikepedia shows the 2006 7300 model selling for $440 new.
Blue book on bikes is just a rule of thumb guide, as local markets will vary. That said, Bicycle Blue book lists the value of a 2006 Trek 7300 bike as between $88 and $138. That said, I would pay a little more if I knew the seller, and was confident he or she could vouch for the maintenance history, or if the seller could show key wear items, like tires, chain, cables and brake pads were very recently replaced and the bike is tuned up and ready to go. If those things have not been recently done, plan on replacing some of all of those items very soon and factor that into the asking price for the bike.