There is an OHS Law/Issue with buying second hand regs - the supplier is supposed to fit a brand new Safety Valve. So - make sure the second hand reg has a new safety valve fitted.
Trouble is, this usually pushes the price of teh regs up and makes the second hand regs unprofitable/not a good buy compared to brand new ones.
I dont know the price you are paying - but if u shop around you can get Brand New Harris 601 Dual Gauge Regs for 80.00ea with a 12 month warranty - i sell them.
Hope this helps you out.