The cell-SELEX method works using the hypothesis that by starting with your aptamer library you can select a more specific pool through selective enrichment. Thus, aptamers are incubated with normal cells and the non-binders (supernatant) are collected, this is followed by incubating with cancer cells and taking the binders.  Typically, the differences between the cells are highlight. This can include different levels of expressions in receptors. Thus, the aptamers selected could target a receptor present on cancer cells but not on normal cells. This of course can be different for each aptamer. see the attached publication.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3559206/ 

Thank you @Mohammed wehbe.