eBay parts?

[metaphasty]

I really couldn't find much around debating the merits and/or legality of sourcing raw materials and COTS from eBay. I understand that there has to be ready availability to the majority of teams as per the rules, and someone has mentioned that eBay could be contentious in this regard as items go out of stock quickly.
However, considering there have been plenty of said items available from multiple sellers, and for a reasonably long timeframe, would this concern be unfounded? McMaster is listed as a legal distributor despite the fact that it only services American teams, which is quite frustrating when they have parts that I can not find elsewhere, let alone locally in Australia (albeit it can be argued that the majority of teams are in America, it just seems quite unfair). Plus, eBay has the advantage in which some parts can be found so, so very cheap.

TL;DR: are (legal) parts off eBay still legal despite that they're bought via eBay?