Not sure you’ll get 50%. In my experience the longer the cable the less to 50% of retail you’ll get. You can always try there for a start but you might sit on it for awhile. I just picked up a used 2M set of Frey 2 speaker cables, mint with all original packaging from an authorized Nordost dealer at 42% of retail and being out of state no taxes. My guess with a 5M set you’ll be 42% or less, maybe closer to 35%. Six months ago from another Authorized Nordost dealer I picked up a used pair of Frey 2 2M XLR again mint with all packaging at 32%. Same dealer picked up a used Valhalla 2, 2M power cord with all packaging, the wood box at 33%. All three recent Nordost cables were from Authorized dealers, mint, which can probably get more than a private seller. My guess is you’ll be closer to 35% of retail on hi end/priced cables. Hope you get more but it might take awhile.
Frey 2 5M speaker cables $5652 retail. I’m going to guess you might get around $2,400 if you have all packaging and no issues.
Good luck.