Wow, D2, I remember that game
Picty is right in that when we started playing Diablo, it was totally reasonable to keep items and trading within the clan only. Main problem was that hacked / duped items were rife, and we were amongst a minority of players who would use genuine items (this was the driving factor in establishing the clan).
But in D2, hacked / duped items are less frequent, and the scope of the game (and perhaps possible penalties for being found out?) seems to have stemmed the dodgy items. Only thing to watch out for is lots of super-rare / too-good-to-be-true items, they are very likely duped (even if the present owner believes they are legit). In the end, asking people not to trade outside the clan would be contrary to clan spirit - the whole point was to make things more enjoyable (which D1 hacks certainly weren't). I'd just suggest using an ounce of common sense and judgement when trading, and if the offer is just too good, heed the alarm bells.
As for trading on Ebay... that's a tricky one. During my MF'ing phase a couple of years ago, I considered buying + selling items, but I could never justify spending actual money on fictitious items. Shame I let a full Tal's set expire on my old sorceress :/ I think i'm probably with Picty on this one - Ebay is a step too far. If there is some ultra-rare item you are after, consider whether the clan as a whole could supply the constituent parts (eg. runes for recipes / upgrading).
At the end of the day, if you are looking to obtain some of the best items in the game, and Ebay is the only way to accomplish it - it's not a million miles from the old D1 days of duping yourself a set of gear. The rarest stuff is meant to be unacheiveable for all but the lucky few, and should remain that way.