I give them a choice
I tell my clients right up front that I'm not a retailer. I offer to source the products they need for a given project and either give them the specifications and let them shop it or I give them the option of ordering through me. I will mark up 10 to 20% and sometimes more depending on the amount of trouble I'll have to go through to get the item. The customer will usually ask me to handle it, but the moment they balk at the price I quote them, I remind them that they are welcome to handle the ordering on their own, provided the item they source is to the agreed upon specifications.
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