Last year I purchased a used PST HP120 that was out of hydro and vis for $50. It cost me $20 for a hydro, VIP, and fill.
The PST HP120 is a rather tall tank. For the majority of my local dives, I actually prefer my Worthington X7-100s and PST HP100s.
How bad do you really need that tank?
How motivated is the seller to get rid of it?
In judging the value of the item, it all comes down to supply and demand in your local market for used steel scuba tanks.
If you really need a tank and you were going to get a HP120 anyway, then you'd be saving a great deal of money by buying this used tank vs. a brand new one.
If you don't really need another tank and the HP120 wasn't really what you were looking for, then you could always make a lower counter-offer ($75?) and see what happens. If someone else wants to pay more for it, good for them.