I've learned a lot from other members on other threads concerning shipping. I would stay away from UPS. Damaged goods, overbilling etc. Having said that, some members here have had nothing but success with UPS so...I would say the majority suggest you stay clear of them. My seller used USPS and everything was fine.
As to being ripped off, I think the real trust is between buyer and seller, less on customs, broker etc. I would think that would be influenced by what you are having shipped, brokerage fees might fluctuate between a 13lb pre amp and a 150lb amp or a set of cables vs a pair of Mirage M3si...but it's all moot if the seller receives and banks your money order and you never see product.
I find the Audiogon folks to be well written, educated, professional types who take this hobby and community seriously. I would feel out the seller, look at his feedback, ask a few questions, the usual precautions.
Otherwise you will see, these are solid people.