EBay Global Shipping Program is a very convenient way to sell internationally. Unfortunately, they "broke" it recently (it became a mess, as I realized trying to sort out a potential international deal. EBay support agents were just as mystified as I was, if not more than yours truly). It's possible they may have fixed it, but I'm still working to cure my PTSD after that experience.
Trying to sell internationally independently is even more problematic. One exposes themselves to the intricate and mysterious ways of local customs, which are wildly diverse country to country, probably office to office, and even customs officer to customs officer. The item, if by some miracle survives the "feetlong drops" implicit to air transit (and understand, I've successfully shipped instrumentation to Europe or Asia, glass-faced or aluminum-faced McIntosh audio components to Europe, Japan and so on, so it's not for lack of gusto or skill), can then be charged a customs amount difficult/impossible to estimate before the item is sent, and may be sent back if things don't align properly. I wish it'd be easier, but all of this is mostly outside of the control of the seller (and the buyer, for that matter...). So international selling of instrumentation or, really, any sensitive electronics is not an easy endeavor. Under eBay's GSP it used to be easy, and one hopes they figure it again and fix it, so it'd be a solution again.
I am not opposed to trying international transactions, but they are an entirely different ball game than the domestic ones.
Ebay will not fix GSP (Global Shipping Program) as it was a service provided by an external company, they have got rid of it in the USA at least, in other places it may still exist ATM.
The replacement in the USA is EIS (Ebay International Shipping), which is supposed to be internal to ebay, EIS dislikes TE & some other categories of items.
As for the different shipping quotes, this will be partly down to the seller, some of the price will be the normal cost charged by the seller, for domestic shipping to the reshipping warehouse (this may also be part of the item price if they include it that way). When listing the seller get a choice of weights/package size, this will be used to determine the international part, the seller may have clicked the wrong weight choice, which is a reason why they can differ a lot.
For the strangely cheap quotes, this may be due to seller listing in wrong category, e.g. TE books, the price may
or may not have a maximum limit for categories like this, ebay can suggest an incorrect category when listing.
Or the seller just selected whatever weight/price the ebay algorithm suggested, of course the useless ebay algorithm can't tell the difference between a handheld TE and heavy rack TE from the same make. Same as the flawed algorithm can't tell the difference between low end TE and high spec TE from the same make and gives incorrect suggested values. Same as the algorithm giving gibberish nonsense
instead of well written descriptions.
These algorithms will be used by sellers with no knowledge of what they are selling, or lazy sellers/flippers.
David