The Downtime rules are an abstraction meant to give you Teams and Buildings that will grant bonuses to your Downtime actions. For example a Storefront grants a +5 bonus to the generation of any Capital the building already generates. What you seem to be looking for is a mechanic to create a "business" which does nothing more than occasionally sells expensive stuff so you can get more money for it.
Step 1: buy a cart
You don't need fancy mechanics or a business. Most market stalls are nothing more than temporary structures like tents, carts or a wagons. If you buy yourself a simple cart and drag it around with you there's always a stall you can sell your wares from. It has the advantage of being portable so you can take it with you anywhere throughout the campaign.
Step 2: put ranks in Diplomacy
There is an alternate use for this skill called Bargaining. First you successfully appraise the item you have to sell. Once appraised you hook buyer. Finally you set an asking price, bargain back and forth, find an over/under price the buyer's willing to pay, and shake on the deal. The actual rules for Bargaining can be found here.
On a final note, try not to overthink this. If you want to be a merchant as well as an adventurer, tell your GM. The two of you can hash this stuff out and if you want to make a formal business out of it use the Downtime rules.