What kind of money do you want to make? Is there money to make a few hundred bucks here and there on craigslist? Maybe. Are you going to be able to quit your job? No. If someone returns a tank you sealed to petco again you will probably get fired. I am not sure if you could get in trouble with the manufacturer either. They pay claims on leaking tanks and if they found out it had been resealed it could cause issues. If the person gets a $2,000 property damage claim denied over your work they may come looking for you. If you are looking at this as a tank maintenance or installation business they will definitely come after you. I never look at a business as sustainable if it has to live off of the inconsistent garbage generated by another business. Eventually people want to get paid for that raw material (trash). This is also something that takes a lot of time at scale and has a niche customer. Whenever I end up with a suspect tank I put it in the yard and keep goldfish, natives, or killis.
If you like building tanks here is a job in a tank manufacturing plant. You will learn more about making tanks than you ever dreamed of, and you can easily claim to be a master tank builder on MFK.
Spectrum Brands