see how bmx blocks are here - they need a backing plate with a bolt and that has the nut on it to go onto the caliper
i just can't see how yours attact :? they don't do a plate like with MTB's that you can change just the block - i maybe wrong but it's just what i can see
One of the advantages of cartridges is that the brake setup isn't lost when the blocks are changed. The plate stays where it is. Just slide out old rubber, insert new rubber and slacken cable.
some of the blocks from back in the day had a metal bar mold in the brake block with bolt screw in it - then you just have a nut and washer on the other side of the caliper - cheap way of doing it
the trouble now is...
there is better pads made nowadays and stop better and last longer
the one's from back in the day didn't always work so well - last as long - and now can be brittle / crumbly - but they are what we had back then an so that's why we look for them and need them for the builds
good luck with sale - but as i think we have found out, it's only going to be for MTB's unless some one has a plate to hole them in