Actually, it doesn't really matter. We've seen several people put the same pin (for example, the Elsa PTN pin) up for auction and trade. While it doesn't necessarily "snipe" customers, it really depends on your view point. Personally, I think it might undercut each other because the people bidding might try to win each of the auctions and not be able to put the same pins up as their bids, especially if the auctioneers have the same type of wants (which most do - e.g. Tangled, Frozen, Up - the popular movies/themes); but it really is up to you.
As for sales, unless you post in that person's thread and snipe their customers in front of them, I don't think it's bad form to post your own sales thread. There are multiple people who have the same pins, so you can't really "wait" for one person's sale to finish when it may never truly finish (sometimes sales threads go on for months!).