Likely no need to reflash it, as long as it's displaying video properly.
I'd say the trouble is the Device ID is not in the ATI drivers, it has a Device ID other than the PCI retail mac version.
It's fixable, I made drivers for OS 9 for all the other AGP G4 Macs( iBook G4, Mac Mini G4, eMac G4), with the Radeon 9200.
I don't really want to do tech support for eBay sellers, but I do want to help our members.
To get the device id, boot into Open Firmware( Command+OPT+O+F ) and type this:
dev agp/@0 .properties
Or if you have OS X installed just look in the ASP.
Armed with the Device ID, you can make the needed edits to the ATI OS 9 drivers.
Now we need Resedit with Forker,
The 2D Acceleration and 3D acceleration where a matter of replacing the 5960 device ID's with the Device ID we want to use. I.E. 5961 or 5962 in three files, the ATI Graphics Accelerator( Two resources we need to change Accl one ID 4 GraphicsAccelerationR6, find 5961 in Hex, should be just after 2C00 in hex, change it to 5962, find the next 5961, again just after 2C00 in hex, change it to 5962. Next open the intr resource and open ID 0 GAInterfacePro, again search for 5961, just after 2C00 and replace it with 5962.
Close the driver and save it.
Open the ATI 8500 3D Accelerator, open the Data Fork, open ID 128 "Data Fork". Search for 5960 just after 2C00, replace it with, you guested it, 5962. Find the next 5960 after 2C00, replace it too. You should find two total after 2C00.
Close the file and save it.
Open the ATI Resource Manager. Open the Data Fork, ID 128, and replace only 5961 after 2C00 with 5962.(Six of them I think)
I think that was it, maybe I should only change the 5960 in the data fork of the Res.Man. that come after 2C00, I'll have to check that and see if things work better.
If you run into trouble, just ask questions, and we'll see if we can get you sorted.