I personally would bank it and open it once you get above level 30, which you will probably do in the next week.
other parts are good, but this is a bad piece of advice. only open lock boxes at lvl 60.
the rewards scale to your level. opening it as a lvl 30 may give you a relic item, which you WILL replace after a few levels (as an example, the lvl 50 relics have much worse stats than the lvl 51 greens)
as bonzodog said, you can get currency, gear, mounts, cosmetic items, consumables.
the best thing that i ever got from a lockbox was a relic rune, the worst thing i got was a cosmetic piece that i already had.
lockboxes are a total waste of time, and i lost more plat on buying keys for 10 lock boxes (boxes were drops from mobs) than i would have made if i'd just sold the boxes, instead of opening them.
I agree, either hold it until 60 or, look on the auction house for the price. Even free members w/o loyalty can buy and browse, just not sell there.
So to sell it, say you saw the cheapest one on the AH was 150 plat. sell yours for 100 to 125 plat in the trade channel COD mail it so they have to pay for it to get it out of the mail. Right now the plat would be of more use than that lock box. Even at 60, unless I need a piece that MAY be in there, I sell them.