" They are not including the necessary sales tax. "
How is a buyer supposed to know they are to pay taxes if it isn't mentioned anywhere in your listing?
"eBay CSR response is to include a request on each listing that buyers do not pay until an invoice is sent. This seems ridiculous to me. "
Again, how are the buyers supposed to know that they need to pay taxes? What's ridiculous to me is that you think buyers are supposed to be mind readers. If you want buyers to pay taxes, you must include it in your listings.
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Non payers should get unpaid item strikes and be added to block bidders lists. Period.