Just paid to have a sub shipped to me. I ended up buying a brand new box from the mfg and sending that to the seller so he could use it to pack and ship the sub to me. FedEx Ground will transport items up to 150 pounds and this sub was at the limit. The only reasons I didn't ask that it be strapped to a pallet are that the seller didn't have a pallet, the cost was very high, and the new mtg's box contained protective wood panels. If the speakers are medium to large, buyers should anticipate the shipping/insurance costs will be in the hundreds of dollars.
To price shipping, you first need to decide how you plan to ship the speakers, i.e., by a common carrier like FedEx Ground or UPS and in their own boxes, or strapped to a pallet and shipped by a freight company.
The pallet approach with freight shipping is by far the safest for large heavy items, but it is expensive.
I would then look up a couple of estimates based on the item size/weight and both near/far ship-to destinations, so you can
provide buyers rough estimates of what they will be paying. I would then clearly state in the ad that the actual shipping and insurance costs will be the full responsibility of the buyer.
I would not sell to somebody who does not want to fully insure the shipment. Speakers are heavy, large, difficult to carry and the most commonly damaged audio equipment when shipped.
Therefore, if you can hand-deliver them that would be best, and that is the only way I have sold large speakers so far. I am getting ready to post another pair of 100 lb+/each speakers for sale and I will be offering free delivery to three states that I travel to for work, if the buyer is willing to coordinate with my travel schedule. Otherwise I will have to decide whether I will be comfortable with FedEx Ground in the original boxes or whether I will insist on strapping them to a pallet and freight shipping. I would rather lose a couple of hundred dollars because of a high shipping cost than to have one speaker damaged in shipping and a buyer who insists I take the pair back.