I'm not an economist, but it seems to me that the most important "transaction externalities" are the legal, institutional and customary frameworks within which the transaction takes place.

No market is viable without a framework of enforceable laws - e.g. mortgage foreclosures, debtors courts - which immediately introduces all the costs of law enforcement - often largely borne by the state and not the individual buyer/seller.  It has often struck me that the most practical and effective way of halting wild speculation and the myriad transactions over the same set of commodities described above is to introduce e a transaction tax (or stamp duty) which would quickly make multiple transactions of the same asset uneconomic.

However the most important externality is probably trust - the belief that what you are buying is worth it because it is from a reputable source (or famous artist - see example above) and that debts will be repaid.  Why esle to people pay Billions for "financial advice" when all the evidence points to the "dartboard method" of stock selection being just as effective in the long run!

"It's a mystery to me - the game commences, For the usual fee - plus expenses, Confidential information - it's in my diary..."