I'm big on setting the scene with extraneous information, so apologies that I don't get right to the point. I want to ward away calls of "irrelevancy" from the get-go.

My group's Dark Heresy campaign is structured so that our particular cell is one of many in this hive world, and the Inquisitor will contact us sporadically with new information by either Vox, or sending a Throne Agent in person to delegate pieces of their missions that they need help with.

This means that our characters have a non-trivial amount of downtime inbetween missions (at least whenever we finish our missions quickly). During that downtime, we live together in an apartment building in the middle layers of the hive.

I have a 61-year-old Adept who relies on her poor-quality bionic lungs to live (strenuous physical activity is a -10 to Toughness tests), and she recently lost half of a leg. While her ballistics skill and gear is sorely lacking in addition to those things, her mind is completely intact, and I was considering leaving her in the apartment in order to start handling any of the mundane, administrative details of our cell while I play a more able-bodied newbie Acolyte to join in the field. All of us players have backup characters drawn up to insert into the story if we ever lose a character, and I was thinking that my Adept could actually simply not be "retired" but one of the two characters I play, though I'll really only ever have to play one at a time. My GM is onboard with that aspect of the plan, mentioning that it might be interesting that instead of the group having an Adept's knowledge at any given moment, they'd have to acquire as much garbage data as they can, then bring it all back to me to sift out clues or design plans.

Now, the question: my Adept has Trade (Copyist) and Trade (Artist). Her backstory includes being a cataloger of an Imperial library on her orbital platform of a home, and she has used both these skills at a point during the campaign. I want her to be able to use these things as more than fluff; I want to be able to have her use the crafting rules to run a publishing side business, or create portraits, landscapes, and abstracts in order to sell to nobles. The money made from this would be used for the cell, ranging from paying the rent to getting groceries delivered to growing the stockpile of combat drugs for acolytes to take to hiring other NPC contacts for particular skill access, to finally buying Best Quality bionic lungs and leg.

The thing is, there are absolutely no guidelines on selling craft goods or offering services. I don't know how easy it would be to find buyers, I don't know how long it would take on average to find buyers, I don't know what percentage of those buyers would actually be willing to pay me enough to justify the cost of materials plus profit, I don't know how much profit I can be charging. Do you guys here have any suggestions on what house rules we should create in order to streamline running a business in Dark Heresy?

I'll probably update this thread soon with my first design soon.

I had a similar situation when I used to play. My Character managed to take control of a ship, (a badly damaged But still functional ship) after completing our mission the GM and I decided that I would keep the ship, hire a crew, and run small charters. I would use any money gained, to further repair and Upgrade my ship, eventually we decided that I'd earned enough money to be able to purchase a second small ship (roughly 2 years in-game time). Though my character never actually gained any physical thrones, I had gained the ability to 'purchase' any small character based items I would like eg. An apartment of my own, better quality clothing, In your case I think a set of BQ Bionic Respirator units would be a nice achievable end goal.

Quick and dirty rules would simply be that a successful Trade skill check adds a percentage to her monthly salary per degree of success. Failure has no consequence unless it is three degrees or more in which case she has annoyed a powerful patron or broken some obscure law (perhaps a guild stricture).

Though Secutor 00K's idea is pretty good as well. Say it takes 30 degrees of success set over as many game sessions as necessary to purchase a Good Quality bionic replacement.