It was not explicitly mentioned in Season 1 that how slavery business was organized by the guys you mentioned in the question but towards the ending of the season the story slowly unfolds that the guys are part of the slavery business.
Keith Summers: Norman finds a journal belongs to one of the slave and starts reading, from the journal, it appears that Summers secretly keeping the girls in the motel room for business purpose.
Zack Shelby: He's a pervert and a crooked cop using one of the girls as his slave and it was shown when Norman breaks into Shelby's house to find the Summer's belt.
Jake Abernathy: Boss of the business and transports girls from Asia, sell them as sex slaves, this was also mentioned in the same journal Norman reads but did not mention Jake's name.
Why did Keith Summers have to sell the motel?
He did not sell the motel, he lost it to the bank, generally happens when people ignore to pay land taxes. This was mentioned in one of the beginning episodes in a rude conversation between Summers and Norma.