Members on this forum will spend 2K on a gun but seem to always put price before anything else when it comes to air tanks, compressors etc.
I always like a bargain, but my bargain comes from a known source of good or really in my case, new equipment, tanks, compressor, fittings etc.
Compressed air vessels are bombs, personally i won't buy a used, tank, just me, but tanks can be lethal, make sure you are totally confident in who you purchase tanks and fittings from. Personally, there is no one selling a used tank, especially on the internet, that i could vouch for so i wouldn't purchase one from them, they don't even know the tanks history.
All of the certification procedures are there for a reason.
I have my own compressor, but i still took my two scuba tanks that i bought new, never been in the water, mis handled, over filled, etc. to the actual facility that does the hydro test.
Don't encourage or request that a tank be overfilled, they may be tested to 150% of their pressure rating, and you want all the air you can get, but error on the side of caution.
In the end each person will do what they want, don't be penny wise and pound foolish, you are dealing with a potential bomb, treat them with great care.
It is just the cost of safety in this hobby/sport.