Things to look for:
Chopped up silicone from people being careless with scrapers.
Salt creep along the lower trim, if there's a compromise in a seam, it usually starts as a slow leak, and water will seep up the trim and pool there, and as the water evaporates, moist salt is left.
Water stains in the stand, from slow drips - either from bad plumbing or slow-leaking seams.
A few small air bubbles usually aren't a concern, as there is also a layer of silicone holding the glass together, but without me laying my own eyes on it, I can't say for sure one way or another whether it's "ok" or not...
Buying any used tank can be a gamble, the bigger the tank, the bigger the gamble, because the bigger the mess if it turns out to be a dud, and with a second-hand tank, there is NO warranty, so you're on your own if it fails.
New or used, especially with a big tank, make sure you have a water escape rider on your homeowner's or renter's insurance. Most policies won't cover a spill and the ensuing damage unless there is a specific rider on the policy. Don't find out the hard way.
Jenn