Go visit the guy with the tank for $400. Try to get a general idea of whats in there. Some typical equipment you can ask about are:
Lighting - Type, age, and wattage are important (T5, T8, or T12 flourescents, halides, or cheapo flourescent)
Tank - Inspect it for cracks and defects
Stand - Inspect for water damage and bowing of any braces especially if its wood
Heater - 300watt minimum. again, age is somewhat important here
Skimmer - Does it have a protein skimmer? If so, is it still functional? Dirty is fine, but broken is not
Canister filters - Does it use canister filters of any kind? Again, inspect for functionality
Sump - Does it have a sump for filtration instead? Better than a canister if it works
Powerheads - What type, how many, what age, and what flowrates or model numbers
Copper - Was the tank ever treated with Copper based medications? Did the owner use tap water to fill it? Inspect the silicone seals. If they look blue-green (kind of like the statue of liberty) dont use the tank. The value of the rest of the equipment still may be over $400 (see Liverock below) but the tank will forever be toxic to corals and some inverts.
Live Rock/Base Rock - By far and away the most important of all would be the presence of any live rock or base rock. If the tank is chalk full of live rock, I'd probably buy it, even if it didnt look all that healthy. LR is by far and away the most expensive part of setting up a SW aquarium. Here in the US you're lucky if you can find it at under $6/lb retail. Often times as much as $10-$15/lb for good show rock. Even base rock (dead and dried live rock) is $2-$4/lb, so if he's got lots of rock, its worth it to buy the tank.
Of course, if its a diseased ridden mosquito breeding pool out in his backyard, I'd stay away...
Hopefully that helps you out with what to look for either buying used or new
