The quoted price for that Dual 2.7 G5 is quite good compared to what they usually go for on eBay. The major issue with the earlier liquid cooled machines, as has been pointed out, is leakage of the cooling system. If you can do a detailed inspection of the CPU area,check for any evidence of past leakage.

I have one of the 2.3 GHz dual core models, the fastest of the air cooled, and it has been great. Performance is very comparable to the 2.16 Core 2 iMac.

Keep in mind that the 2.7 has a slower memory system than some of the later models, but often makes up for it by raw processor speed.

In short, the price is good and if the cooling system is in good shape...go for it.