Selling Location & Purchasing Location

Dear Community,

Currently, we have one location for purchasing and selling. We are thinking of separating to create a location for selling & another location for purchasing.
I just want to know what changes this would have on us. Correct me if I am wrong but we will need to create a transfer order from purchasing to selling, or else we won't be able to get ETA dates (available to promise feature). Do transfer order usually come before or after purchase order? or does it not matter?

Is there anything I should be aware of? I am trying to also set up Distribution Resource Planning to do the demand & supply planning for two locations. I am a bit confused...will the supply plan create transfer order for selling location and then create purchase order for the purchasing location?