OK for a gun used quite a bit with low expectation for remaining value.
Not OK for a brand name gun (Boss, Purdey, etc) which high value is likely to stay over time.
I think twice before buying anything below 25mils, and probably not at all if at 20 or below.
It's always possible to dent barrels, and low wall thickness makes the denting probabilities higher as well as the consequences more dire. Not to mention the safety part that should be OK as long as the thin spot is much ahead of the fore-end.
Something not to forget is that corrosion can be nasty under the ribs, and that it is not possible to measure thickness there.
The thinner the tubes, the scarier that becomes.
Best regards,
WC-