Can you specify what fins they are exactly, or post a photo? So far I do not know any Beuchat Mundial Elite fins (only Beuchat Mundial Elite guns). Currently they only have the following freediving fins in the catalog:
If yours are the carbons, then I'd be interested, but I am afraid it is not the case - they should be more expensive than $150 (new). But if you are selling the Sports or Competitions or an equivalent ancient plastic model, then your price if far too high - Sports and Competitions are sold new in retail for 65€, it means some $80 (for example
here). I am afraid you'll have hard time to sell used ones for $100.
If you are selling them just because the blades are too hard, then you may consider getting replacement softer blades - the Sport plastic blades are quite soft and inexpensive (cost only 20€ per blade). Or if you are looking for an upgrade, then look for fiberglass or carbon blades. As for fiberglass, the
Matrix (made by Waterway) blades are used a lot, and as for carbon, the cheapest I saw are the ones from
Pierre van Eecke - they cost only 149€ the pair. Before buying, you should check with the supplier if you can mount them into your foot-pockets though. Beuchat pocket mounting holes are slightly different than Omer or Sporasub - you may need to drill new ones, or buy blades with corresponding holes, or no holes at all.