I guess the topic of quality sellers on Ebay has been one of the most popular here on FPN and other forums. There are a lot of very good and honest sellers out there. I have bought from them over the years - pens as well as Zippo lighters. The sellers I tend to buy from most often are well versed in fountain pens even if that is not their main business. I look at the other items the seller has or has had for sale. If they seem to be doing a brisk business in pens, have a stellar rating, fair prices, good photos and descriptions and will politely and helpfully answer questions, I'll buy from them.
When I started buying, collecting and selling pens, I did not know what to ask. FPN forums were invaluable to me and still are in that regard. I love dealing with other FPNers - they are a very helpful and resourceful group. Dealing on Ebay requires some diligence and care. Often a deal that appears to be too good to be true isn't.
I have made mistakes when buying, usually my fault because I was rash in my decision, did not have sufficient information or thought I was scoring the deal of the century (greed?). Once in a while a pen was not as advertised but I have never had a seller who did not offer a discount or make it right. Yes, I compromised a few times.
I am able to do most run of the mill restorations myself so when I buy a used pen I usually can calculate if it is worth buying and fixing it up. When I send a pen out to a restorer, I ask that they let me know the cost once they see the pen but before they start work. That way I am only out the cost of postage if the work is more extensive than I can afford or want to go through with. I may have misunderstood the OP's original comment, but it seems that the cost of repair/restoration of the pens was a surprise.
The cost of postage, especially with tracking and/or delivery confirmation has made mailing international very difficult. I like Priority Mail for domestic mail and currently also use it for International. Flat rate small box is about $19.50 for Priority Mail International. On a pen under $100, that is expensive. Plus, the tracking and delivery confirmation complications make it even more difficult to sell globally.
Hope this wasn't a ramble.....