I've bought rare PvP set items from blue bags and the blue bags themselves from other players at some point when I didn't have the AP to buy them myself. I've sold rare set items from the same bags (that I bought with my own AP) to other players for gold as well.

Edit: I think the blue bags are still the best rare loot reward system in the game. You get to choose the armor type and the gear slot of the item (to some extent), and the trait is fixed. The items are rare but not impossible to obtain and they can be traded, the set bonuses are interesting but not overpowered. However, the bonuses could have been a bit more specifically designed for PvP.

If people are still selling AP right now I hope they have all of the Monster sets they want. Personally I am being a miser until I see just how much AP these pieces are going to cost. Selling AP right now is like shooting yourself in the foot. Come that first week or 2 of Thieves Guild everyone will want more AP!.....especially if they half decent traits on them like the Impenetrable pieces I hear are on PTS now! Lol

I've bot the Akaviri Chest motif since there's no way to otherwise get it and I'm not great at PVP.

spend it all on siege and repair kits.