I'm unsure of the details, but this is the principle:
You would need to setup as a reseller, buy said item(s) without paying tax (on account that you are going to resell them), then sell to your customer(s) with an appropriate sales tax tacked on to the price you charge. As mbhunter states, the sales tax should only be paid once by the consumer.
I believe that you cannot avoid setting-up as a reseller, as you would be expected to collect sales tax on the items you sell. In the case that you do pay sales tax on an item and simply pass it on to the final consumer you would at least have to keep a record of the fact that the sales tax was already paid. In the case of a markup, I believe that you have to charge a sales tax on the mark-up. This gets you back to setting up as a reseller, as you generally do not want to expose the markup to your customers. Rather, you would want to buy it tax-exempt, add in the markup and charge sales tax on the entire sale.
You may be able to charge them for the service, instead of a markup, though I am very much unsure of this.