I believe it's still a very relevant/useful addon. I haven't gone looking for things, but I believe that there are still, in new content, items that are available from obscure vendors that get tossed on the AH for gold, especially if the concept is expanded to things that are available for currency (rep, pvp, etc) and still tradeable. Regardless, I still see people putting items from e.g. Dealer Rashaad out in Netherstorm on the AH for stupid amounts of gold. I call it out when I see it in trade chat, but that's only so much.
It's not something I can do; I tried looking at the Lua book a few months ago, and I can't make sufficient sense out of it. (fibro brain fog sucks)
I'll add a +1 to this request. Would really love to see Swindler Preventer resurrected or, if that's not possible then maybe someone can create something new to replace it. It's really a very unique addon that I've been using for years (from it's original author/creator) and it's saved me from spending silly amounts of gold on items you can purchase from vendors for a few silver countless times!