Disposing of chemicals in the UK can be a challenge.
I had some 12 year old stale Leaded Petrol, over 40 litres of it to be exact, that I needed to dispose of safely. When contacting the council you would have thought I was trying to dispose of radioactive waste ! Eventually I found out that the local waste disposal amenity would accept the petrol provided it was given to them in proper approved petrol cans. And they get to keep the petrol cans !
I was not keen to give away all my nice petrol cans so I approached my local garage who service my car. They have a licence for oil and fuel disposal. They have to drain car tanks when the owner has put the wrong fuel in at a garage. The licence and disposal costs is expensive but they said they would take my 40 Litres no problem. As it turned out they put it in Jerry cans for use in the Garage Kerosene heater !
Other chemicals that are uncommon in domestic situations can be a total nightmare as they are often classified as Industrial waste, for which disposal is normally chargeable at quite high rates by specialists. Sadly some total Bast*rds sometimes dump such chemicals in the countryside or pour them down storm drains where they poison the local streams, water courses and rivers.
Sometimes people give away 'out-of-date' chemicals rather than having to pay significant sums for correct disposal. I am not at all surprised to hear that the seller wanted rid of so much flux liquid. Better $5 in his pocket than $100+ for specialist disposal costs, if such exist locally.
A good find, but also a bit of a liability in terms of getting rid of it all safely.
Fraser