Hi ! As a longterm online seller, I think when you say tax you mean customs fees. Look at your sellers feedback and pick carefully. TALK TO THEM ahead of time and let them know that they should be careful with the value on the customs label because most probably do NOT know that you pay tax based on what they put. Many won't lower the value for you, some will. The value on the green post office ticket that gets filled out is STRICTLY for customs, its not insurance and it should NOT include shipping costs. So if you pay $8 for an item and $12 in shipping, the value should be $8 or lower, not $20.
I always value it at MY cost or what I would consider a replacement cost. But that's me.....I'm a single mom and I'm cost-concious.
If a seller is obnoxious or won't work with you, don't buy from them. It is never, ever worth it. There's always another seller. Worse comes to worse, find a super nice one to buy from and have your other pins from other sellers shipped to them(in the US) and have THEM(nice seller) bundle 'em up all for you as a lot with a lower value. I'd do it for my customers.
Good luck!