What are the good registrars if I will either sell my domains with or without websites later on?
I've been reading some discussions here, looks like Godaddy, name.com, namecheap, dotregistrar are some common registrars. However, I've read some articles where people said some registrar will make transaction very difficult. What do I need to look for if I want to sell my domains with or without sites in the future?
Thanks!

I would recommend eNom, it is convenient to push your domain to another eNom account.

Yes, such registrars as Godaddy, Name or Namecheap are quite good and have domain auctions where it is possible to sell/buy domains!