Galvanized steel tanks, properly cared for, will last a lifetime... whether or not it is "used" should be irrelevant to the price. However, there is always the concerns regarding condition of the cylinder and condition of the valve. Given the hazards, these issues should be addressed with due diligence.
Here is our "best practice" suggestion for trading among individuals in "previously owned" galvanized steel or unpainted aluminum cylinders. Start with current retail price of the cylinder, deduct the current cost of a hydro and a valve. This establishes the "asking price" and then negotiate until both the seller and the buyer are unhappy about the price being paid.
The seller is responsible for delivering the cylinder with a fresh hydro (so they can document that a cylinder fit for purpose was delivered) and the buyer is responsible for the dive shop VIP and supplying the valve (since rebuilding a used valve and replacing the burst disk is often not cost effective.) Thus both the seller and buyer should have no safety or liability concerns. There are also no issues surrounding the diving qualifications of the buyer.
For example, HP100 w/boot and valve the new price without shipping is currently about $350; less the price of a hydro typically around $35 and a Thermo valve retails for $50, that yields an asking price of $265. Once the actual selling price is agreed regardless of the time remaining on the hydro, the seller retains the valve then arranges for and pays the cost of the hydro at the DOT approved facility of their choice. The seller delivers the cylinder to the buyer, then the buyer arranges for and pays the cost of the VIP and valve at the LDS of their choice. This works even when the transaction involves shipping. However, if the buyer and seller are able to agree on a single local dive shop to handle the hydro and VIP then the transaction is fairly painless. The seller delivers the cylinder to the shop, pays for the hydro. The buyer picks up the cylinder from the same shop and pays for the VIP, a new valve and a fill. In all cases the dive shop becomes responsible for determining the diving qualifications of the buyer.
Of course in circumstances where the buyer is knowledgeable enough to properly inspect the cylinder themselves, or buyer and seller have a shared history of trust, these precautions might be unnecessary.
