Yes, the Century guns can be a crap shoot, but if you get a good one, $500 is a bargin for a shooter. They are in the $420+ range in SGNs. It is worth the extra money to be physically able to check it out.
Another area to check. (real) L1A1s are inch guns. Metric mags will work, but may not lock tightly in place. Check fit and function of both types of mags, as metric mags are more available and cheaper. L1A1s have their bolt hold open pins demilled. An STG58 part, available from DSA, easily retrofits. A true L1A1 also has sand cuts on the bolt, and a folding cocking handle (which is MUCH better than the STG58 knob, IMO).
Check the gas system components. Does it operate smoothly, wear, etc. if you can, have your dealer break the gun down for you. My century L1A1 was a bitch to open until I cleaned it up.
A word of warning, if you buy this gun and like it, expect to see a DSA (or comparable quality smith-built FAL) in your safe.