I'd buy a used 40D if if it had less than 20,000 shutter releases (actuations w/e)
23,000ish is probably fine. Nikon will say 100,000 to be on the safe side anyway, so you might get 150,000 for all we know!
Its kind of like overclocking a processor (although to a lesser extent with different cpu stepping G0, G1 etc for the Q6600) some overclock well, some don't even if they are the same stepping, but just a different bit of silicon wafer.
So e.g. my E6600 gets from 2.4Ghz to 3.3Ghz, but others can get up to 4Ghz, some can't even reach 2.8Ghz.
So e.g. Some Nikon D200's can get 150,000 shutter releases, but others can get over 200,000, some can't even get to 125,000.
(see what I did there
Canon EOS 5DmkII + BG-E6 + Canon EOS 40D + BG-E2N + Canon EOS 33
Canon 17-40 F/4L
USM + Canon 24-70 F/2.8L
USM + Canon 28mm F/1.8 USM + Canon 70-200 F/2.8L
USM IS + Canon 85mm F/1.8 USM
Canon Speedlite 580EX II + Canon Speedlite 540EZ + 2 x Nikon SB-80DX
Cactus V2s Wireless Trigger - 5 x
Cactus V2s Wireless Reciever