Is all PC 133 SDRAM DIMM 168 pin suitable for Macs

Buffered, registered, and ECC SDRAM are slightly different from each other, but their purpose is essentially the same. Each type is designed to verify the integrity of the data before it's passed along, so that errors can be detected and corrected (if possible) when found. While buffered and unregistered memory are typically used in servers, some home PCs have ECC memory in them. Macs don't support any of these types of memory, so you must look for unbuffered, unregistered, and non-ECC SDRAM. The most obvious physical difference is an additional chip on each side of the DIMM. A compatible 512 MB DIMM for your G4 will have (8) chips on each side, for a total of 16. An ECC 512 MB DIMM will have (9) chips on each side.

Your G4 Digital Audio requires PC-133, whereas the slot-loading G3 iMacs (350-700 MHz) only require PC-100, but can use PC-133. The auction that you linked only indicates that the pair of 512 MB DIMMs were pulled from a 500 MHz iMac, but doesn't identify them as PC-100 or PC-133. Since you need the latter type, you need to know before bidding. A simple means of identifying PC-133 would be the individual memory chips' speed, which should be printed on them. If -10, -8, or -08 is printed on them, the SDRAM is PC-100, whereas -07, -7, or -7.5 would indicate PC-133.