I buy and sell Macs on eBay every now and then and would never buy an iMac G3 or G5. The G3 because they're extremely outdated now and a lot of sellers tend to think they're worth £100-200, but the G5 because of the relatively high number of hardware faults. If you do a 'spares or repairs' search under iMac, most of them will be G5s. The G5, too, is often overpriced by sellers.
However, if you were to buy it, you'd need to buy another with a working screen (you'd probably get one that's had a logic board failure pretty cheap) and perform the 'surgery', selling on the 'donor' to get some of your money back.
It would be very risky, but would be worth it if you were able to pull it off.