The woman sold her means of living (flax, spinning wheel, [yarn] reel) and - taking inspiration from Steve Gardham's contribution,
[I'll sell my rock, my reel and tow
My gude grey m,are and hacket cow,
To buy my love a tartan plaid,
Because he is a roving blade.]
- maybe the less valuable tow from the flax plant,too. And that woman sold her horse and cow, too. So, I think "rock" might have originally been "flock", to complete the spinning motif.
Having done that, she could well be destitute. "Rod" in one version presumably is due to someone assuming the reel is a fishing reel, which makes no sense in the context, but at least it is alliterative.
Begging plus prostitution makes sense to me.