Some people are fond of saying that a certain piece of gear is "bombproof." The USGI Lensatic Compass is such a piece of gear. Mine is dated 12-59 and it's still going strong. I have had mine for approximately 25 years. I don't even think anymore to calculate how long it has been, I stopped at 25 years. But the compass is older than I am!
The one thing you should probably steer clear from is the Tritium version of the USGI Lensatic Compass. You know, the type that "glows on its own" without an additional light source. They are definitely awesome pieces of gear and function the same as the regular USGI Lensatic but they go for two times the cost of the standard model, sometimes 125% or 150% more, depending on who you are dealing with. Most of the people on E-bay and forums are not listing the date of manufacture so you are probably just wasting your money as the Tritium has a life expectancy of 10-12 years.
While these are excellent compasses, top-notch gear, if you don't get resupply from the great helicopter in the sky, you might want to stick with the standard, rugged and accurate USGI Lensatic Compass and purchase a Petzl Headlamp, battery caddy and extra Energizers for it with the spare loot you save.
USGI Lensatic Compass = Excellent lifesaving gear.
USGI Lensatic Compass with Tritium and manufactured one year or less from time of sale = good deal if you can justify the additional cost for the ability to see the compass in darkness without a flashlight.
USGI Lensatic Compass with Tritium and no date of manufacture available or over a year old = if the individual or dealer still wants top dollar, avoid them.