I am a serial watch flipper, and have had more than 60 watches coming in and out of my collection; so I have a bit of experience in this context. Most of these watches eventually went to other collectors locally and from across the Causeway; but there had been some that travelled to Europe and the likes. It helped that I used to work at that part of the world, so I could pass on to friends as transit point; and they in turn assist to have COD whenever possible.
1) I have used various couriers, although some have had delays. But no rule of the thumb here, it's more like taking your chances. (Just like I have seen in-coming parcels being diverted to totally different addresses [as seen on the web-tracking page] by local courier companies and complaints from both mine and the seller's side thereafter fell on deaf ears. Thankfully, the seller had insurance and refunded me the monies paid.)
2) Best to take all-risk insurance to cover for any damages or lost in transit. However, IIRC, the coverage differs from courier companies to companies; and some would outright reject the service if they were not comfortable to send it out. Why so, you'd ask. Well, I was informed that based on claims experiences, they have had customers claiming to send original timepieces out but thereafter complained that the recipients received faked watches; and the courier companies were none the wiser. I once had to send a RM40k watch and the courier company informed me that they could not do the courier if I wrote in the details as such on the CN, and advised me to send it labelled as just watch. After all, he reasoned that only he and me would know the real value of the watch; so that would be relatively safer. If I was sending vide courier overseas, I would only do so if there was a much higher premium in the sale price compared to selling locally, or if the value of the watch was in the lower ranks such that it would not be a big hit to the pockets.
3) Payment was mainly through Pay-Pal for overseas buyers. Note that they do hold the sum for a period; and in case of disputes (whether genuine or otherwise), they could effect a return to buyer if proven that seller was at fault.
4) GST for the countries differ from one to another, and is borne by the buyers. Normally, they would have effected in the full GST while negotiating with the seller.
Hope this helps.