NEW YORK CITY — Marcus & Millichap has brokered the sale of a four-story apartment building in the Park Slope neighborhood of Brooklyn. The 17-unit property, which is located at 400 6th Ave., sold for $6.5 million at a capitalization rate of 3.4 percent. The sales price equates to $604 per square foot. Matt Fotis and Matthew Shapiro of Marcus & Millichap represented the seller, a private investor, in the transaction. The buyer was also a private investor.