Hello again :-) After not having posted in the BR for several years, I now need advice again!

I will be semi-retiring in a few weeks, and intend to fill my time by becoming a (small-time) car trader. After many, many years of 'contributing' to the car trade as a buyer, I'd like to give it a go from the other side of the fence....

One question I do have concerns the process of handling V5C registration documents for vehicles in stock. As I understand it, the batting order goes:

I buy a car from a private individual (the registered keeper on the V5C), we both complete and sign section 9 (the yellow part) - which I give back to the seller to send to DVLA.

The rest of the V5C is kept by myself until the vehicle is sold - whereupon:

The buyer fills in the 'new keeper' section 6, and signs the right-hand declaration in section 8. I give the buyer section 10, then send the rest of the V5C back to DVLA.

My question is: do I also sign the left-hand part of section 8? This is labelled 'Registered keeper' - but the V5C will still be in the name of the person from whom I bought the car. As I'm not listed as the registered keeper, do I just leave it blank when I send it to Swansea?

Hopefully, a seasoned professional can advise!