Well, when I buy an item in the $100-$500 price range I usually say something like: "If you wouldn't mind marking down the value of the item a bit, it would help me avoid some taxes, but I understand if you choose not to."
Is that being a wanker? I've never forced someone to do it, and with certain items or very high dollar items I actually ask that they declare the full amount and insure it to help avoid any problems with me receiving the package. I would also never back out of a sale because someone decides not to under declare. That is their choice, and I respect it.
I've actually had a couple people under declare packages they've sent to me without me asking for it. Luckily enough this has worked in my favour, but it is the reason that I now ask that they declare full value when it is an important item.