I can't really see the benefit in this.
So I pay ~$30 for a Do-It lead mold plus $12 shipping. Then I send it back to Do-It incurring another $12 shipping and I will most likely get at the most $15 for my mold.
For that I would just keep it or sell it on EBAY or the forum. I don't see Do-It paying the shipping for the mold so I think that is out as well.
Like others have said sell them on the forum or on Ebay and let the buyer pay the shipping.
I don't do soft plastics and I believe those molds cost a bit more but I still do not see this happening with those molds.
Then of course with lead molds you have a whole host of modified molds which to me are really worthless. I say that because we so often do not know what the modification is/was for. We also do not know if the reason for selling the mold is because the mod didn't work or it ruined the mold.
Often people modify a mold to suit their needs/wants which may be totally different than mine for the same mold.
Like I said I do not do softplastics and maybe there would be some benefit with the more expensive molds.