My tertiary experience was so long ago (1980s) that there were no OERs!
And the textbooks were not necessarily affordable, but seemed part of the overall cost of attending university. The only options were used textbooks, which sometimes I used, and re-selling textbooks back to the store for a pittance of their initial cost. I recall some consideration by my or other peers about not marking up books so their resale value would be better.
A reverse example: many times I have researched courses “dreaming I could attend” and bought the textbooks they recommend without registering because the cost of tuition and resources combined was not feasible.