Welcome and thanks for your question!
You are allowed to pass the sales tax that you paid on the parts purchase onto your customers.
If I decided to mark up the cost of the parts, would I still just pass along the sales tax I paid, or charge sales tax on the new cost?
You would still just pass along the sales tax paid. The parts are incidental to the cost of the service performed.
If you aren't charging your customers sales tax and you purchase an item without paying sales tax, then you as the purchaser are responsible for self-assessing and paying in use tax on that item. You can still pass along the cost to your customers in the end.
Great, thanks for your help in this matter, One quick addition, do different rules apply if I just stay as a sole prop or LLC?
No the rules are the same regardless of what type of business entity you operate as.
Great, thanks again DanielleCPA!
You're welcome! Have a wonderful evening!
You as well!