I've used this plenty as a buyer in the past and it works. Mostly. Just be aware that in order to cut costs, eBay will use the cheapest and slowest shipper going and tracking can be very hit and miss. Just over half the time, transatlantic flights were routed via CDG airport in Paris before being put on another flight or train to the UK. That added an extra week at least to the shipping time because packages had to clear customs in France before continuing. This was even when the UK was in the EU. Once in the UK, more customs and more delays.
The other issue is that your package will be handled A LOT, which increases the risk of loss or damage. I had one or two packages almost completely destroyed. One was even left out in the rain at one of the depots. The fault was partly inadequate packaging at the seller's end and also negligence on eBay's shipper in the UK. eBay just stiffed the seller for the lot, which was a bit much in my opinion but they are completely in charge once the package gets to Kentucky in the US or wherever the UK GSP depot is and the seller doesn't get a look in. I'm not confident eBay will protect the seller too much in the event of a complaint.
Plus point: It was at most half the cost of USPS or RM and you didn't have any customs worries. If you are going to use it, err on the side of over-cautious packaging.
