Ian
Reading your original post, if you simply want these unwanted plans to go to a good home and are not too bothered about remuneration for them. why not make a list that interested people can pm you for, and then ask for perhaps £3 or £4 per plan to cover p&p and any surplus give to a suitable charity? That way, the plans are hopefully going to good homes, your storage problem is solved, a charity benefits and you will feel 'good' about it all. If you went via an online auction site, inevitably someone somewhere will be buying a plan to resell at a profit.
There is nothing wrong in your selling on an unwanted plan to someone else as Telstar (Tom) has written. A problem would occur if you copied a plan/design and then sold the copies to new customers.
Selling (or passing on) an unwanted secondhand plan to someone for their practical use is no different to selling a secondhand car, model boat , r/c system etc. in my view.
The NMM do however sell their plans with all sorts of caveats – such as they can't be used to create a new 'model plan' without an additional fee being payable which makes the exercise uneconomic. i possess a complete set of plans for a Ham class inshore minesweeper from NMM. I can use them to create a model, but cannot use them to create a master hull, and then sell mouldings from the plug without paying an additional, and substantial fee. In other words, they want a share of any profits, but on the other hand, i also have a complete set of plans of the Belgian equivalent (6 feet longer) with no such restrictions as they came from the public domain in Euroland.
Hope this helps
Paul Freshney