We suggest using good faith best efforts to do your due diligence, which is probably exactly what you will have done anyway as part of your research into identifying each item you purchase. We recommend you keep written records of your due diligence in case it is needed in the future, but we will not be auditing your decisions unless independently requested to do so.
Report the individual item not the lot and include the details as you can – i.e. the artist’s name and details of the sketch, the seller of the lot, and the date of the acquisition. And again, if in doubt please contact us.