You are possibly overthinking this one.

Your client as premises occupier will have arrangements for deliveries, and as you indicate collections i.e. rules for drivers & vistors.

Whether this is your customer (buyer) arriving in their own transport or transport arranged by your customer why do you believe you would need to conduct checks?

What you should have from the client is a set of site rules to pass on to any purchaser who indicates they wish to make their own collection along with any arrangements for doing so e.g. scheduled collection slots, opening hours, PPE requirements.

Thanks for your response. I might be overthinking things as you say. In some cases we arrange the haulage on behalf of our buyer and obviously in that case we do need to do our due diligence I was just wanted to be sure before assuming we had no responsibility for our buyers own hauliers. 

It came about after a discussion regarding our hauliers and their accident/incident reporting requirements. We currently don't ask them to report to us if there are incidents on our client's site. Which I think they should be doing but I don't think that will extend to our buyer's hauliers, because as you say they will follow site rules and report to the site.