RAM - what is ECC?

First: ECC is Error Correcting Memory and is it the Wrong type for your iMac. Standard non-ECC memory is what you want, it will not necessarily say non-ECC, it just won't mention it at all.
Other words to avoid in memory are: Registered, Buffered, Parity - all of these words also refer to ECC RAM. So entry number 2 is definitely out.

Second: I would not go to eBay for memory. Choose a reputable dealer who guarantees compatibility with your iMac model. There are a number of different versions of PC100 and PC133 RAM that are not compatible with your machine. One thing is the module has to be low density (16 chips on a 512 module).

Thanks
Trevor
CanadaRAM

Here in the UK I use Crucial for all my memory requirements - here's their page for your model;
http://www.crucial.com/uk/store/listparts.aspx?model=iMac+%28G3%2D500%29

As you can see, it does not support ECC. PC133 is easier to find than PC100 these days and is perfectly compatible.

ECC Memory;
Short for Error-Correcting Code memory, a type of memory that includes special circuitry for testing the accuracy of data as it passes in and out of memory.
(from Wikipedia)

A 64-bit wide memory needs 8 extra data bits to store redundancy information to do single-bit error detection and double-bit error detection. There is substantial additional logic required to determine, on the fly, whether an error has occurred, which bit is in error, and hold off the processor/cache while the correction is made. This logic (to do the error correcting) is usually on the processor board, but the extra bits must be carried with the regular data on each module. You need enough bits to point out which bit is bad (6 for the data bits plus one more to cover the parity bits, too) plus one more bit for double-error detection.

The whole process can go faster if the memory can "hold" the data in a register while the correction is computed, so the modules are designed somewhat differently. They are not interchangeable with consumer memory.