For sure the Amazing Spiderman 50 and the Atom 1 are very solid books with widespread collector's appeal. The Space Adventures 33 is the first appearance of Captain Atom and a tough book to find. Brave and Bold 29 is the second appearance of the Justice League, so there is some great content here.
Condition plays a huge role in the value of comics, so arm yourself about how various defects affect condition - i.e., creases, tears, stains, loose staples, etc. Always look through the books to see if any coupons are cut and check to see if the centerfold is intact (and/or loose).
Never, never take the first offer or fall for high pressure tactics!!