* Region 2, "Savazushul"
I looked at the members of three merchant companies:
Searing Company - out of 93 dead members, 14-18 engaged in posthumous trading.
Whirling Guild - out of 14 dead members, 4-5 engaged in posthumous trading.
Soaked Guild - out of 6 dead members, 1 engaged in posthumous trading.

Gulo Calmdumpling: struck down in 109, traded in 127
Jath Wheatbrushes: struck down in 90, traded in 115
Tedaz Godvine: struck down in 90, traded in 127
Ithru Fishyawns: murdered in 88, traded in 127
Osnun Planterror: struck down in 101, traded in 102
Ngokang Jackalpaint: struck down in 101, traded in 102
Nikom Stilledpleats: murdered in 149, traded in 169
Mudi Squirttrick: died of old age in 139, traded in late spring of 139 - might not be posthumous
Rayali Birdsstood: devoured in 159, traded in 169
Alatha Silkengilt: devoured in early summer 168, traded in late winter 168
Pevo Joyousfurs: struck down in 182, traded in 184
Ushav Shellsculpted: died of old age in 192, traded in 210
Rofa Loneplays: murdered in 186, traded in 194
Onuth Miledented: murdered in 198, traded in late autumn 198 - might not be posthumous
Conu Primmurders: murdered in 224, traded in 240
Onuth Graspedsnarled: murdered in 226, traded in 240
Upu Revereddreams: died of old age in 236, traded in late spring 236 - might not be posthumous
Astri Prophetknead: died of old age in 243, traded in late spring 243 - might not be posthumous
Utast Closetsilvers: died of old age in 193, traded in late autumn of 193 - might not be posthumous
Ebpel Sandalmined: died of old age in 163, traded in 175
Islas Jackhails: died of old age in 185, traded in 194
Tunem Bandteacher: died of old age in 185, traded in 194
Opra Violetspoons: died of old age in 179, traded in 194
Lastta Faintedbother: died of old age in 200, traded in 210

* Region 3, "Osathadar"
I looked at the members of one merchant company:
Sizzling House - out of 24 dead members, 3-4 engaged in posthumous trading.

Nimma Presenttaker: died in 176, traded in 185
Ducim Wheeldangers: died in midspring 306, traded in early winter 306, reanimated in 319
Led Galleyguard: died in 365, traded in midsummer 365 - might not be posthumous
Serid Breachedjail: died in 364, traded in 365

This happens in every world I generate.
DF version: 47.04 (linux)