I find that characters offering 750 sparks for slaves to be not very beneficial to peruse.
It really depend on the way you are playing, where you are in the game, with which character, the difficulty setting for obedience, etc.
Selling D+ for 750 sparks is the thing that permitted me to take off with Johny in normal start/normal difficulty, in addition to living in the slum (for 0 rent), lowest living condition (0 additional cost) and eating every day at Virgin's Hip (1 spark/day).
Let's say you train her in 20 days in this condition that should cost you something around 100-200 sparks, giving you a ~500 bonus by D+ and if you min-max even more you can achieve greater benefices.
This is good for the start but the game incite you to sell higher rank as soon as you can afford it.
If you have a house and a rent to pay, therefore higher living condition (so additional expenses), an assistant and cook food at home then yes selling D+ is likely to be a net loss.
Once I sell to all of the customers who are offering 3500 sparks for B+ rated slaves will I eventually run out of them?
I've read that they should ask again at some point but that it take time, in my current game i sold D+ in the outcast to two customers at the beginning of the game and they still don't need a new one 400 days later so i am not sure if they refresh but by the time they do (if they do) you will most likely not sell D+ anymore.
You can ask the guild for a contract, you will have an advance and a bonus if you deliver quickly.
I personnaly don't do contract but the option is there.
Or you can sell to sub-purchaser at the guild, not the best price but nice for a quick buck and it's unlimited, no prerequisites, he will give you a fixed price and you accept or refuse.
Finaly you can sell (indefinitly ? didn't tried) to quite a lot of NPCs, like the doctor, the bartender, Madame Jofre at the Salon or any NPC who let you ask "Check for preferences", they will tell you what they want.
At the point you are trying to sell B+ i would recommend you to sell them to the Guild Auction of Branded Slaves (every 10 days, don't confuse it with the slave auction where Farid sell slaves tho), you can make a very good benefice there.
I have heard that selling 3 slaves in a single quarter will open up the ability to meet with higher up officials
That is what i have heard aswell but i can't answer that since i don't sell to them anymore, maybe it's a fixed number sold or a specific rank with the faction.
I sold 1-2 D+ in each quarter at the start and then switched to the Guild Auction.
If you click on your character portrait > Objective tab > "I need to work my reputation", you will see your current reputation with each faction color coded and a hint about what you have to do to gain more like "i need to sell a C+ to them".
I'm just trying to make sure I'm not failing at something without even knowing it
This is higly likely to happen, this game as a 89° difficulty curve and you will certainly make mistakes in order to learn what is efficient and what isn't. I recommend to save often.
You will not run out of customers but you can quickly go broke if you don't pay attention to your expenses vs gain.