How do you bid on them ... am I missing something ...

I was over cap so was that the reason I didn't see a way to bid on them?

during the season you use the sign free agents button in the upper right task bar of your team.

during the free agency period you click on advance and then use the bid on free agents button on the top left to open up the bidding pages.

I need a SS but wasn't able to get one during FA. Now, no SS show up on FA list. However, when I choose the search button and select SS, there are quite a few listed with N/A under team name at the end of the list. Who are these guys? How do I sign them? They all show as undrafted. Why aren't they listed as FA?

maybe check your filters, if you accidentally left minimum 90 strength on (or something) likely no SS will qualify

Alternatively, you can just change the position of a FS or LB.

N/A guys could be retired as well depending on which tab you're on

How can I tell if they are retired? Most of these players are 23-24 years old with ratings from 60's to 80's.

if they go 2-3 seasons without being signed by any team, they'll usually retire.

I think those are "filler/fake" players no ? I'm playing with my custom roster and I see bunch of those guys but I can't sign them nor any other team can. Who are these guys ?

Could be the draft class too.