I can't see that it is a problem for you, but it would be for your customer if you did not deliver as agreed.

I must admit I have never seen an enquiry about dropshipping from the point of view of the supplier...it is always the "middleman" wanting advice, and he is usually advised not to do it.:^O

Are you sure he wants to buy from you on ebay and not privately?  I have never heard of two sellers advertising the same goods.