Wet Seal Inc. said it offered to buy 508 stores from women's clothing chain County Seat Stores Inc., which is in bankruptcy proceedings, for undisclosed terms. Wet Seal would convert the stores to Wet Seal or Contempo Casuals stores. Closely held County Seat referred the offer to its advisor, Blackstone Group, and hasn't responded yet. The purchase would increase Irvine-based Wet Seal's number of young-women's clothing stores to about 900. It also would give Wet Seal a valuable expanded base in malls. Wet Seal wants to make another acquisition after its successful purchase of Contempo from Neiman Marcus Group in 1995 for about $1 million in Wet Seal stock. Wet Seal shares gained $1.875 to close at $20.875 on Nasdaq.