Snow Leopard does not work on G5 iMacs, so you shouldn't have tried installing it.
You got the wrong product on eBay and will be unable to use that version of Tiger due to it being Intel-only (The MacBook Pro install discs used an Intel-only version of Tiger, and your processor is PowerPC).
You should contact the seller and inform him that he sent you the wrong product so you can get a refund or the correct discs. If he gets uppity, take it to eBay buyer protection so you can get your money back. And then leave him a nasty review.
In the meantime, do a clean install of 10.3 from your iMac install discs, since that's all you're going to get to work right now.