would i be libel

[wa4cch]

i have a questition if i ran a web site that sold used ham gear and on my site
a ham that has been known to sell stolen radios and also takes money and never delivers and i knew of his past history and i still allowed him to advertise on my site could i be held libel if he sold more radios and failed to deliver since i had been warned of his scams ??????

thanks chuck