The 29th annual Lawrence Art Auction, to benefit the gallery program at the Lawrence Arts Center, 940 N.H., will feature works from more than 250 artists in both silent and live auction formats. This year’s event also includes some changes intended to answer concerns that artists’ works were undervalued. A minimum bid of 50 percent of retail value has been set on each piece. “I felt we needed to protect the value for artists,” says Rick Mitchell, the center’s gallery director. “This isn’t a garage sale; it’s an art auction.” The auction raises between $85,000 and $120,000 a year, or 40 to 45 percent of the gallery’s operating costs. Silent auction works are on display in the gallery through the beginning of the auction on Saturday. The arts center’s doors open at 6 p.m., with the live auction at 7:30. Tickets are $25 in advance at the center, or $30 after 6 p.m. Saturday. This year’s featured artist is Dave Van Hee.