Do NOT but anything from Sera if you're trying to bankrupt her for the sake of acquisition. Recycling an old post of mine:
Farm Grey goos on Tarkus, make sure your INT is high so you can reprogram and take grey goo samples to sell. As mentioned above, make sure you have Colenso's quest done so you can sell items to him at higher prices, also Anno is good for this when on your ship (store Anno won't just buy everything).
If you're a weirdo like me, you can set Steele up to take in multiple sustainable revenue streams:
1. Sera's Salary (will require you to be Sera's sub and complete a certain event with a certain number of points -
https://wiki.smutosaur.us/TiTS/Sera) and will require you to check on her on Tavros station once a week to get your pay.
2. Lane's payment (will require you to be Nice personality and overcome Lane's brainwashing and completing a certain event -
https://wiki.smutosaur.us/TiTS/Lane)
3. Become a Nyrean Royal (will require you to complete the story event on Myrellion in the Deep Caves -
https://wiki.smutosaur.us/TiTS/Queen_Taivra's_Palace). Once in a while you will be given a Gem Sack you can sell when you visit the palace.
You can also whore yourself out a Beth's Busty Broads -
https://wiki.smutosaur.us/TiTS/Brothel_Mistress though this will take some time. I've gotten to a point where my current Treated Steele will take in an average payout for 3000 credits just using her mouth. Each sessions takes 2 hours with a 15 minute cool down. Kat the brothel mistress goes to sleep at 6:00 and wakes up at 10:00 so you can cram in some serious cash if you work at it.
Addendum: There have been additional ways to generate credits since I posted this.
Uveto: There is a stadium where you can bet up to 10,000 credits on a race. If you like to save scum, this is the place for you. Additionally, you can farm for Savicite but depending on your level and gear might be a high risk venture for certain players.
Breedwell:
https://wiki.smutosaur.us/TiTS/Breedwell
Fight and defeat Doctor Lash -
https://wiki.smutosaur.us/TiTS/Doctor_Lash
There should be a guide on taking Lash in this forum
...also, save before attempting to do so.
You'll probably want to min/max stats and get somewhat geared for this fight.
Just save edit and/or cheat.
Optimal vendors to sell to for maximum profit:
Anno after getting her to join your party and Colenso after completing his mission involving a certain rogue A.I.