I participate on block swaps here and two other boards. One board has a quilting nanny and if the blocks are off by more than a quarter inch or if there are other problems she sends them back. She also has a tutorial to help folks learn how to sew scant 1/4" seams, get perfect points, etc. The other board is "anything goes". The last swap was for 8 blocks and I ended up with 5 usable. With postage as high as it is these days, it's not worth it to me to get unusable blocks.
Actually postage is what has made me re-evaluate participating in swaps at all. But I do enjoy they when they work right.