$20 for the results of bash one-liner? (by my count there are far more than 40 left)

    for i in `egrep '^[a-z]{3}$' /usr/share/dict/words`
    do
      whois $i.io | grep -q '\- Available' && echo $i.io
    done

Heck, there's still a few two-letter .io domains left

    for i in {a..z}{a..z}.io
    do
      whois $i | grep -q '\- Available' && echo $i
    done

    iz.io
    jy.io
    kf.io
    kq.io
    lq.io
    qf.io
    qg.io
    qj.io
    qo.io
    qs.io
    qy.io
    uj.io
    yd.io
    yl.io
    yz.io

But, yeah, I agree, .io choices are getting a little thinner than they used to be.

Selling the remaining list is quite a smart idea. it obviously has value even though it would be relatively simple to figure out the remaining free domains on your own.