From Sept. 3 to 6, more than 30,000 people are expected to attend Baltimore's 29th Summer Antiques Show. Billed as the largest indoor antiques show in the U.S., over 550 international jewelry, art, and antiques dealers converge. Highlights include a 70-dealer antiquarian book fair and a slew of silver specialists. ''There are incredible values -- you're not paying retail prices,'' says dealer Gus Davis, adding with a laugh, ''retail is for rednecks.''