Do I bury them to avoid them spoiling?  Thanks - unsure since they spoiled in my inventory.

Turnips will spoil when the next Sunday comes around.

You could try leaving some turnips on the ground where Redd's tent will appear. I think it will cause the ones the tent smushes to appear in the lost and found, where they will not spoil. (it worked in previous games).

Otherwise, you could leave the turnips in a town that has a different date than yours if you wanted to keep the turnips longer than a week.

Also I believe that if you time travel they spoil too? but yeah basically the goal is to sell them and make profit before the week is up, when joan comes back to sell you more.