How does sales tax reimbursement work for reseller?

Hi, I'm hoping someone could help me understand what I can/can't do. I just started a new small online business. I applied for and got my sales and use tax permit (Texas). I have a couple of questions:

1. If I purchase items from another seller (another retail store) and pay sales tax at the time of purchase. I can get reimbursed for the sales tax. Do I have to approach the seller about getting a refund or can I just claim a credit on my tax filing?
2. Can I get a sales tax reimbursement for inventory I purchased before my sales and use tax permit was issued? In other words in anticipation of starting this business, I purchased some things for resell while still in the process of setting my business up.

If anyone knows the answer to these questions I would be very appreciative. I've searched online but I can't seem to get a clear answer on these questions