You know you wont screw it up, I know you wont screw it up but you will get someone who just bought a cheapy from ebay and read a little about it and thinking they are an expert.
Im sure they will have an onsite mechanic who would put their own through it or lt RAC do a test on it.
If i was selling my car, i wouldnt let someone stick a diag machine on it unless i knew they were insured just incase they screwed it up.