I get asked to fit 2nd hand appliances all the time I always say that we are not allowed to fit it without seeing it working before it was disconnected. ( The truth is I just dont want to risk my career fitting a 2nd hand appliance) I have used many 2nd hand parts to get things going in the past dont do it know after someone put a complaint against me to the landlord for using 2nd hand parts (I think that was even plumbing side not gas) so after having 2 days of grief and almost losing the company the contract I stopped. All payed off though next time I got called to the house for a boiler fault on a freezing cold day. They said "can you fix it?", Me "oh sorry I only have a used part in the van that would get you going and I dont do that anymore as I had a complaint about it", Them "But its going to be cold waiting 3 days for a part", Me "sorry bye"
To the OP I have been in peoples houses where they have nothing and I need to turn their boiler off and they have been in tears, I felt horrid but if you have to do it you have to do it. There is no point putting yourself into the firing line if something goes wrong. Most people will plead poverty but when they have to will find the money even if the family get involved.