Hmm, selling diaries and notes must be in vogue or something...
But anyway, you can trade with like 90% of NPCs, but proper traders are not that many in act 2, even if you're considering the whole map. The 4 in the driftwood square, the paladin leader, the rogue dwarf in the tavern, the arena trader, Baran on the inn's 2nd floor, Tarquin, Almira. I pretty much never needed to trade with anyone else aside from these. But you also have Jahan, Ryker, doctor Swann, and maybe a couple odd traders here and there.
Rule of thumb: Once you've located the key traders, just bump their attitude to 100 and never trade with anyone else. No reason to sell stuff to some random guy...
I can't recall any quest in Act 2 that requires you to have any letter on you though. In most cases you just need to read them and the quest would be updated. Unless you sold the thing without even opening it up and reading it...