From my experience you should expect to pay up to 10% depending on the state you live in. I shipped a dac to Italy for updates/service and when it crossed the border on the way home it was reported. A few months later I received a bill from the state requesting 10% of the value I declared when shipping it.
Fortunately I had written all of this (returned for service) on the international waybill when setting up the shipment. The tax collector could have easily seen this but they prefer to make details YOUR problem. The waybill was used as a means of avoiding the tax when they came to collect.
If it were me I would purchase with an American Express card to ensure there is automatic buyer protection and then contact DHL (or whoever you prefer) and arrange for the shipment yourself. At least that way you have control over the information given (declared value and replacement insurance). The carrier representing you should also be required to verify the item exists (is actually in the box) in the case the seller was planning on sending you a case of cinder blocks.