Personally for me, if I'm trading a $5 card for a $5 card that I already have, it's not worth it. It costs $10 to send it out + I get another member to send it out for me so I usually like to give him a profit. So on top of $10 shipping, I would add anther $5 to $10. Also it's a pain for me to go up to his house to drop off the card. It's not worth the hassle for me. Now, if we're doing a bigger trade and I'm trying to fill out some stuff to balance out the bv, I'll gladly take some cards I already have to work things out.
*cards take of space. Space is limited. If I'm taking up space in my room for cards, it better be something that I don't have.