I prefer to do blocks swaps etc with people in person, thru quilt guilds, friendship groups, quilt classes ... that kind of thing. More likely to agree to patterns, fabrics, colors etc. when in a group face to face.
The ones I've done online have been a bit disappointing and maybe others might have been disappointed in mine too. Also I get stressed if I'm not on time. I don't need more stress, LOL.
And as someone else mentioned, I don't want to pay to mail them.