I grew up with this sung as "I'll sell my flax, sell my wool," which always made perfect sense to me. She's selling her spinning. True, it doesn't have the same connotation of desperation, but it does have the image of waiting instead, which I think is equally relevant.

Anyhow, about two years ago, I ran across this song in the BBC Sharpe's Rifles series mentioned a while ago and the line was "Buy my love a coat of steel" instead of "sword of steel" which places this at the English Civil War at the latest, as that was the last British war where armor was commonly used. The series is circa the Napoleonic Wars, so the retention of that detail works - it's like us still using the word sword in that line, despite the fact that swords are obsolete for warfare.