I made this graphic a while back but thought I should probably expand on it a bit in a less sarcastic form.
The debate over whether rats should be sold in petshops and whether we should purchase them has raged for at least as long as I've owned rats (15 years as of 2021, which is frankly terrifying). Without some significant changes to the law, it'll probably go on for years after I die, with my skeleton still participating in the debate.
I'll say it now, although you'll probably be left in no doubt as to where I stand on this particular issue by the end of this: I don't agree with animals being sold in petshops.
Have I bought rats from petshops in the past? Yes. Do I regret having some lovely rats in my life as a result? No. Now, do I regret financially rewarding the petshop animal trade? That's a big fat YES!
Most of us have been in this situation at some point: there's a cute face in a petshop, probably being kept in unsuitable housing with little enrichment - possibly the last of its group to be purchased. Maybe the rat looks sad, or maybe even ill. We want to help them, to ensure they end up somewhere better. This is the immediate picture in front of us - and purchasing that cute face is certainly a solution to giving that rat a good home.
It's easy to think that only a heartless individual could deny that sweet little face a safe home. For whatever reason, we've made a connection: it looks just like one of our past heart rats; if we don't buy this rat then we don't know what will happen to it; we've looked it in the eye and it's tapped the morse code for "help me" on its tank!
Whatever the reason, that rat is pulling at our heartstrings. And we know that there's a solution, and we can offer that solution! We can save this rat!
This one rat.
Petshop rats have to come from somewhere. I know, stating the obvious, but it's a salient point when you look at the bigger picture.
I'm not going to guarantee that every single rat in a petshop has been bred in a rodent farm/mill (This is an old article but one I often link, if you're interested in a glimpse into how rodent farms work/worked a while back).
I'm not even going to suggest that everyone who breeds to stock petshops has malicious intent or money at the forefront of their mind. I'm sure some people genuinely want to promote rats as amazing pets, and genuinely want to do their best by them.
There is one obstacle which I've never seen evidence of any petshop breeder overcoming: record-keeping.
Yes, you can select from unknown or unestablished lines - it's happened fairly regularly with newer varieties or lines that would significantly benefit the fancy/pet rats in general - but generally speaking, those lines are worked with by people who are selection pros; people who know rats extremely well because they've spent a hell of a lot of time around them, in whatever capacity.
Good breeders need to keep records - however in depth, and it does vary - because the whole point of breeding to better a species (feel free to envision your own version of 'better') is selection. For a good breeder, selection is the basis for every mating decision, every rat they keep to breed the next generation - and records directly inform that selection process.
Assuming the breeder isn't present for the sale of every single animal they've bred to stock a petshop (which is the case for the majority, large chains in particular) - that's already one degree of separation which complicates record-keeping. If you want to find out about your rats' relatives/ancestors, you'd need to go through the petshop to reach the breeder. The chain of information is broken down from the moment rats are handed off to the petshop.