I've gotten a lot of items there for $.99-4.99 that have made me $5 there or $20 here, but in general, it's very hit and miss. You really need to know clothes, books, art, kitchen items, etc. to pick out the few gems in there, as most Goodwill merchandise these days isn't "Grandma kicked off, lets dump her entire estate without looking through it!" but rather, "This is the crap we couldn't give away at our garage sale".
Not to mention, if some ultra-valuable gem comes in, someone else will grab it before you do if it is anything at all obvious. The people pricing and stocking it alone will grab it if it's that obvious.
That said, when I was a teenager and starting out with reselling things, I did fairly well because of a Goodwill. It was in a wealthy, older age bracket neighborhood and the management did not give a damn what things sold for. I'd go in 3-4 times a week and pick up bags of older comic books for $2, a few video games/VHS tapes/records for $.25-$2.00 each, a few books at $.25-50 each, etc. and quickly resell them. I never got rich, but I got a lot of things that made me decent profit.
A copy of the 80s 1984 film adaption for $.25, resold for $25.00 quickly; a few rare NES and SEGA games quickly resold for $10-50 each; some unique (or at least popular) records for $5-25 each, etc. Plus, I got a lot of cool things for myself. I snagged a leather jacket there that was new, with tags from the late 80s, and undamaged other than a small rip on the collar. $5 to buy it there, $20 to get it repaired, and I've had it the last seven or eight years as my go-to jacket.
But all good things come to an end. It switched management and the prices skyrocketed. It seems that since then very few Goodwills are affordable, much less steals. Unless you're getting brand name, excellent condition clothes or a book/movie you really want for yourself, there isn't much picking anymore. Everyone grabs the obvious valuables and everything else is priced so high it isn't worth the gamble. Garage sales are better now.