O2 Sensor Questions

[smartguys]

Hi, I may have posted into the wrong place yesterday, sorry if I'm repeating:

I wanted to be sure I didn't make a mistake:

I see chatting about 02 Sensor readings/replacements. Porsche Dealer pulled code, and told me I need 2 sensors replaced. Parts and labor $640!!!!!!!! What a scam.

I bought 2 on ebay. Very cheap.

They are not custom fit OEM, rather, one sensor fits all -- assuming splicing the wires to the old clip is a simple job.

Question #1: Are these ok to use? The ebay auto parts store, when asked, said yes, as did past customers. I want to be sure.

Question #2: Is there a photo diagram of the replacement process available somewhere that shows how to do this job?

Question #3: Do I need 4 of them?

thanks
Kelly