Selling a litter of pups can easily be classed as a hobby. So the situation is different but of course if you were claiming the vet bills, food etc as expenses then yes you would need to declare the sales as income.
When buying a biz, you can only buy physical things, ie premises, stock, equipment in the customers or contracts are only viewed in relation to the value of that biz.
Customers may leave or go elsewhere so cannot be included as an expense
I worked for a company who were brought out for 8 million pounds, the premises was worth about 1 million, machinery a further 1.5 million but we had a full order book and work totalling 6 million
We were brought by a rival US company, and instantly closed down us down.
Basically they paid well over the top to take our work and kill off a rival company, the two owners were happy as they sold a company valued at around 3 million for 8
The tax man didn't go chasing about overpaying for a factory unit and machinery.
There is more than one way to skin a cat.
Darran