I was thinking the other day about whether it would be a waste of time and money to breed my own guinea pigs and sell them to a store like pets at home or any other pet shops like that.
Pets@home use a commercial supply mass breeder for all their branches; these days it a UK based operation working down to a budget. The branch shops get regular country-wide deliveries but will only keep a certain number out on the shop floor at any time. The rest have to wait in very basic cages at the back. Pregnancies should no happen but do due to mis-sexing at every step of the process starting at the breeders' they do, unfortunately. If the pregnancies are picked up on, then they are dealt with behind the scenes in order to keep complaints from the public to a minimum.
We are talking about hundreds of branches and piggy sales every week.
In the last few years the chain has had a real (but officially not acknowledged) problem with ringworm - we've had 10'000 hits in a year on our ringworm care guide for several years in a row and numerous enquiries from purchasers - and that is just the tip of the iceberg. That has mostly stopped for the last 12 months or so (hits on the ringworm guide have thankfully slowed down a lot and so have the enquiries on here) but it is rather lowering to think at what cost of innocent lives.
Breeding for a quick buck is sadly only going to make a situation that is currently out of control even worse. Because of the money crunch adding to the post-pandemic pet dump, guinea pigs and other pets are being got rid of at the moment left, right and centre with only very few adoptions. When you suddenly need to save money, small pets are the first to go... We've seen it back on 2008 and are seeing it much worse again now.

How we are dealing with the usual post-Christmas 'toy pets' dump in this climate is anybody's guess... It happens every year as soon as the kids get tired of their surprise pets and nobody in the family can be bothered to care and pay for them. Plus the usual pregnancies and surprise babies; especially from unregulated source bought by people buying on a whim without doing their research.
We are a rescue-friendly pet owners place who cares about a species that has a healthy average life expectancy of about 5-7 years but that can live much longer in good care and that have a complex social life. Our piggies are full of personality; they feel pain and neglect no less than us. We are aware that can only ever do our best for the tip of an iceberg with a huge underbelly of sheer misery and suffering.
Sadly, we are already dealing all too often with the messes resulting from the sale of diseased or mis-sexed/sold pregnant or fighting teenage piggies on here, on a daily basis. As you can imagine, the last thing we want is more people adding to the problem; especially sub-standard breeders willing to ignore basic welfare and hygiene and not having the first clue of basic biology.
I have recently adopted 3 piggies that had been found living rough in dog-walking waste land and that took several days to catch. Since one of them was a boar (son/brother to the other two piggies), I am now dealing with 7 piggies due to two pregnancies and consider myself having got off very lightly. The first lot of babies were born in rescue not long after their rescue and were extremely small with their mum not producing enough milk and drying up early. I am hoping that with already quite some inbreeding and three generations involved that there is not a lif-shortening gene in play and have done my best to prevent any further pregnancies where the likelihood of any babies inheriting a faulty gene from both parents will become ever more prevalent if that kept going. But that reflects the reality of what overbreeding pets meeting fast rising living cost looks like at the other end...
Guinea pigs are much more expensive to keep responsibly than you'd expect. A tiny hutch in the garden with vegetable scraps thrown in won't do. Unless you are callous enough to let your breeding piggies die in agony during a birth going wrong - ending up minus mum and any pups to sell; it is not something I would recommend undertaking, especially as guinea pig mums give birth to babies that are a multiple of the size in relation to human babies with the resulting much higher rate of fatal birthing complications in both mothers or dead-born babies/litters.
I appreciate your need to find ways to supplement your income but breeding guinea pigs when rescues have dozens if not over a hundred piggies at not immediate risk of life on surrender waiting lists is not a clever idea. Some small private rescues have had that many enquiries every single day and have also had people sent to them by the local RSCPA - who they are expected to function without the much larger resources of that organisation I struggle to understand. That rescue had to close their doors for any surrenders whatsoever as a result because they could simply not keep going.
My dumped piggy family has been adopted from a rescue with a waiting list of over 100 piggies, by the way. Their place was immediately taken by a bonded group of 6 piggies whose owner on benefits is no longer able to pay for their hay (which is the mainstay of a piggy diet and is not quite cheap).
I hope that this helps you to understand a bit more of the whole situation you were considering branching out into?