It seems (like shown in the screenshot on the other topic), that the Travelling Merchant won't need supplies and gold for producing the goods.
I'm in the last chapter - Travelling Merchant I for me isn't worth the space. My marble manufactories (with all other buildings required to have the manufactory – culture, population, culture for population, supplies, culture for supplies, population for supplies, culture for population for supplies etc.) still produce a little bit more T1 goods/square than the merchant. For people in earlier chapters (which do not have max relic boost nor a Mountain Hall) the building can be quite interesting.
But, if the numbers on elven architect are correct, Travelling Merchant II (for T2 goods) is already better than my crystal manufacturies. And Travelling Merchant III seems to be at least - in regard to production/square - twice as good as my gems manufactories. Of course, I can't replace my whole gems production, but somewhere there will be some space for one or two of these buildings (if I am lucky enough to win them with one or two tries)...
Everything subject to change if I discover an error in my calculations^^