If atheists thinks that there's a small chance that they will turn into theists and be glad to be in possession of a spare soul, then they must think that theists value spare souls. So it would seem more valuable to theists, who don't have to multiply the value of the transaction by the small chance.

There are some differences between typical theists and the hypothetical atheist-turned-theist. In particular, the theist has had a lifetime to keep a clean soul. But many theists think they do a bad job. If the spare soul has tail risk value to an atheist, it should have more value to the bad theists. The other difference is that the atheist is not a believer at the time of the transaction. Perhaps the belief of the theist makes it a greater sin to trade in souls.

But it seems like a lot of details have to go right for it to be a better deal for the atheist than the theist.