

The seller thinks the value is net negative to them. The buyer thinks it still has a potential positive value. Both would agree to just hand it over.
Unfortunately, UK law does not allow that. Consideration must go both ways. The simplest way is to sell for the minimum reasonable amount. $1 is traditional in the US. In the UK it is £1. The other commenters link has a good writeup on the practice.
These can get an impressive range to a water balloon. Reduce the mass with an egg and 1/2 mile is likely (just) within range.
I would say stink bombs are even lighter, but I doubt Trump can even smell them, over his own stink.