I've been playing a lot of BC currently and i found there are a lot of items from classic still relevant in BC.
The obvious choices are still the Free action and Living action potions. Not much of a surprise here, they are still as amazing in burning crusade as they were in classic, letting you ignore stuns and snares is good no matter the mode, though they are unusable in arena which probably made more than half the classes let out a big sigh of relief. They are even made a bit better since they cant be dispelled or stolen.
The next item on the list is the mage trinket Mind quickening gem which in classic quickens your cast speed by a third for 20 seconds. Obviously this is one of the best trinkets in classic but it also holds up pretty well in BC, however the stat change in BC does hurt it a little as going from 33% to 330 haste isn't quite the same. Still this one is worth picking up if you can get your hands on one, while farming BWL.
Now for the healers, i'd recommend picking up the scarab brooch which drops in AQ and it applies a shield when you heal someone equal to a percentage of the amount healed, its probably one of the best in slot trinkets for healers in general, since it basically increases their healing by 15%. Obviously however, this is one of the items that's gonna be contested pretty highly since every healer basically wants one.
Next up i'd like to talk about he darkmoon cards, specifically the Blue dragon one. It gives you a small chance to regenerate mana while spellcasting on a cast. Now this is obviously a raid favorite as it lets healers stay topped up on mana while they're healing which is generally the only problem for healers.
The other one is here just for the memes, its the Twisting nether one which gives you 10% to resurrect when you die. While it's not competitive in any way, it can be used in arena, however it does mean you have to sacrifice the benefit of a different trinket which might let you win the fight that this one revives you from.
These are the obvious items that i'd keep my eye and try to hoard up for BC as they're likely to be in high demand, however their prices might fall a bit as there are obviously going to be other comparable options.