how does this seller craft these items?

This is an interaction between bestiary 'create imprint of magic item' and 'split a rare item, half mods on each'

He starts with 1 magic item with spectre. Creates an imprint (craicic chimeral 64c) and then regals it to make a rare, then splits it (fenumal plagued arachnid, 2.4ex)

Not 100% sure on the next part. When splitting, the original piece in the bestiary item slot corresponds to the imprint. The new piece that is pooped out of the bestiary is not. If the spectre is on the new piece, can restore the original with the imprint.

If the spectre is on the original piece, regal it again and split it again.

What happens to successful split : sell in the link you have. He probably augment/regal to prevent people from capturing his market, because need to start with a magic item

What happens to a failed split (spectre on original) : sell the 4B2W 6L astral plate.

Expected outcome: 50% fail, 50% success. On average, two tries will create one 4b2w plain 6L, and one 4b2l spectre 6L. 5ex to create ~7ex of value.

And if he hits an excellent augment or regal (like 100 life) thats even more money

/r/pathofexile Thread Link - pathofexile.com