I have:
. Created a chest with the items I want my merchant to sell, and placed it in the world,
. Created a faction with Vendor parameters, linked it to the chest.
. Created my actor, assigned them to the faction I created, and to JobMerchantFaction.
All this but still no joy. On top of that I don't even want to get into my issues with the custom dialogue. I'm tired of writing out masses of dialogue for an NPC, then having them simply spew default dialogue without engaging in conversation, or the CK recording only one audio file for each response.

Does the "What have you got for sale?" prompt not show up? Dialogue issues are sometimes caused by using the wrong voice type. If your merchant does not have appropriate recorded dialogue, the prompt won't show up. If your custom dialogue is not showing up, the problem is likely related to your conditions. I can't help you with creating audio files because I've never created custom voiced dialogue.