this welder is all over ebay for $665.99 new with free shipping, so just on the welder alone you should be looking at $500 or less since it's used. i have checked craigslist for welders, and i have seen a lot of people selling for a certain amount when i can buy a new one for the same or less. either they are trying to get as much as they can by hoping you don't shop around or else they didn't shop around and just went out and paid retail.
as far as the tank goes, if he indicated that he paid $65-$75 for the tank, then that is exactly the cost of a one year lease on an empty tank, plus the cost of the ar/co2 inside. if i cut a deal with him on the welder, i'd just say keep the tank and let him deal with that. leases here range from $25 to $45 a year per bottle, depending on dealer and length of contract. a fill will be $30-$45.
around here (i assume it's the same everywhere) leased bottles have the name of the dealer stamped into the ring on the top of the bottle. if you look right under the large threads (about 3" diameter) where the safety cap screws on, there is a raised ring about an inch wide. if this ring is completely smooth and has no markings on it, you should be looking at a customer owned cylinder. if the ring has a name stamped into it, then it belongs to whoever has their name is on it. i can't gauge the size of that bottle - it looks to be close to 3' tall? around here the standard sizes are 20cf, 60cf, 120cf, and 280cf - sizes vary slightly depending on what gas is inside. the 120cf and 280cf are considered commercial sizes and are very rarely customer owned. the 20cf and 60cf are quite often customer owned. yours may be an 80cf? i'm not familiar with them, so i can't say where they fall. i do know that a 60cf will generally cost $100-$150 for the bottle.
as far as other issues when buying a used cylinder, if the identification ring indicates a customer owned cylinder, you also want to make sure they have the transportation cap that protects the valve. if not, deduct $10 to $15, because you will have to pay that to get a new one when you turn in/refill the tank. one other poster mentioned the tank date. i've used gases for years, and i still have never been able to decipher all the stampings on tanks. if the tank has no physical damage to it (welding arc marks will instantly void a tank) and it still has gas in it, then it is likely to pass a test. i believe the test is $10 for 10 years, so it's not a huge cost to have it recertified. if you own a tank, you're going to have to pay those fees eventually.
around here people/places don't generally have their own tanks refilled anyways, they swap their empty non-lease bottles for full non-lease bottles. some places break down the $10 recert fee into each tank exchange and others will just bill you every 10 years if you stay with them as a customer.