Well, I might have read the license wrong. If I read again now, I understand it is partially open, quote from their website:
There are 2 important things: OsmAnd uses third-party code only with permissive licenses like (LGPL, MIT, Apache) and OsmAnd code itself copyrighted by OsmAnd BV is distributed under GPLv2 license with the exception of distributing it on Google Play markets without permission. That provides a possibility to build any application for personal usage or build an opensource application under GPLv2 license (cause OsmAnd is not a platform and all the code built on top of the core must also be licensed as GPL).
Sorry, can't give solid tipps for the UK as it has been a while ... but is "craigslist" still a thing? (It was controversal, I remember.)
For the "ebay" alternative: Maybe if you search for "small ad boards" or "notice board" in your reagion, something comes up? In the countries I have been in recent years, there are such boards online and also in supermarkets, community centres and sometimes near the council administration office.
Furthermore, look out for "charity shops", "antique shops" or the mentioned thrift stores. Just randomly searching, I found this article which might give some ideas, too: https://www.co-operativebank.co.uk/content-hub/articles-and-advice/the-uks-thriftiest-cities/