I have trouble keeping up with the eggs part of the eggs & milk trade, seeing as how 60 eggs are required for the noodles -> spicy feast -> Explore Dark Caves adventure (as close to daily as possible)... though I'm nearly always maxed on milk.
So I instead find myself often doing the Magic Fruit exchange, but that's un-sustainable because of the limitation of receiving 1 magic fruit per day.
I find fur to be the limiting factor on trading wool, hide & fur for building materials, and rat tails to be the same for the exchange with Belladonna.
IMHO, considering that trade route upgrades give an early boost on the number of trade tickets collected but once they've been exhausted the collection goes much slower, I think that the Royal Exchange should randomly give 1-3 trade tickets with each trade... perhaps 60% chance for 1 ticket, 30% for 2 tickets, and 10% for 3 tickets (average: 1.5 tickets per trade).