I have a couple dozen transactions on GunBroker.
Pay attention and you will be fine.
"Buyer pays actual shipping" is not code for "whatever he means", and to think so is ridiculous.
Most of the time I've seen "Buyer pays actual shipping is on an auction from an individual or a small shop that does not have a "shipping calculator/estimator linked to their auction.
If you'd bother to call or email, you'd find most of those notices means you'll actually pay LESS shipping when others would charge a flat rate.
Now, like I said, you do need to pay attention.
I did have one schmuck who tried to charge me "Overnight Express" shipping when it was not necessary.
I bought a Daisy Red Rider BB gun from a seller who listed overnight shipping ONLY, and expected me to pay $45.
I asked him why he wanted to ship overnight his answer floored me.
He said, "This is a dangerous weapon. I will ONLY ship overnight, and ONLY to an FFL!
I told him he might as well list me as a non-paying bidder because there was no way on God's green earth I was going to pay for overnight shipping.
He FINALLY agreed to ship it standard post, but insisted that it go to an FFL.
My FFL did not charge me for the transfer, but he STILL tells people about my "dangerous weapon".
Due diligence is required on GunBroker, but if you didn't already know that, maybe you should refrain from such venues.
Having dealt with you, I'm not sure the BB gun is the sangerous weapon. :P