Over the past few years it has become the fashion to sell anything online, from a box of drawing pins to a jet fighter, there is always a buyer who may be interested. the novelty is still there to sell stuff that is unusual or weird, in fact the market is actually better with these strange products.
Old rope has been sold before so there is no reason why it won't sell again. So where do you get old rope from? Many places, an old washing line, harbours are a good place, camping equipment, dog leads, shopping bag handles etc. Rope is used in many places and usually chucked away. Just ask family and friends if they have any old rope lying around in their house or garage, you'll be surprised what you will get. The main thing is not to pay anything for it.
The next step is to take some photographs individual pieces of rope. Try and enhance them by having a background that may lead the buyer into thinking it had its working life there. For example a rope on the seaside shore may give the impression it came from a shipwreck. Or set it with a wild west theme as a rope that has been used for hangin someone. Just think where the rope could have been used and with a bit of imagination, the rope suddenly becomes a special product with a working history; an antique in fact.
Once you have a few pictures, you need to write an account. A few online encyclopedias can give the a good starting point telling the history of rope, this can be a good starting point for your description and work around that.
You are now read to market it, there is however only one place it will sell and that is on a online auction, this it the big business for buying and selling novelties and this is you next step.
Put a reserve on it so you don't lose any overheads you may have had - it should be nominal anyway. Add a little more into the posting and packaging, after all it is your time that is being paid for anyway, but not too much to put buyers off. You can also suggest they can come and pick it up personally if they are locally based.
Once all is in place, it is the waiting game, but don't get too downhearted if there is only 10 minutes to go in the auction and not bids have been made. Many people wait until the last 5 minutes before committing themselves.
If for some reason it didn't sell, tweak your description and re-list it, your rope will sell eventually and you will have achieved something and made a profit at the same time. Why not do it again with another old piece of rope?