I think the $300 price tag is way to much. After all, it is used and who knows what is actually in that tank. Copper for example which would prevent you from keeping inverts. I would only pay $75-$80 for the whole set up. Otherwise just walk away. If they really want to get rid of it they will go for it.
Heck you can get the 55 "kit" at wal-mart for like $150, yes...filter would need upgraded and most get better lights, but you can always pick those up pretty cheap in the classifieds, ebay, aqua-bid...$300 WAY to much
I sometimes buy used tanks and equipment. If it is in great shape I offer approx $1.00 a gallon, if it has extras I look at how much and offer what I feel comfortable with. As example I picked up a 55 with 2 HOT magnums, heater, AquaClear 500, Whisper 60, Whisper 40, canopy, hoods, lights, substrate, and decor for $150. I also picked up a 90 bare and 15 complete for $100.00
It is worth as much as you are willing to pay. Some are tightwads and others more generous. If they need to get rid of it I offer less, if it is something I want I offer more. It's up to the buyer if it is worth it regardless of anyone else's opinion.