I have seen some good discussions on what makes someone a back yard breeder, honestly though its a really personal thing. You need to decide whats important to you, your personal improtant ethics wont be the same as mine or anyone elses. You jsut need to understand what your potentially letting yourselves in for and what you are supporting and decide if your ok with that. Heres some areas to think on
Inbreeding - most breeders inbreed to some levels, however some breeders and people find it abhorant (to differeing degrees, sometimes people refuse to do brother/sister matings but are ok with uncle/neice). Inbreeding is an improtant tool in selecting for good things but done badly will also select for bad things. If you go to a breeder who inbreeds make sure oyu look at how they select there rats, if they are selecting for temprement / health etc then inbreeding is a great tool, if they arent then it carries a lot of risks. Going for a breeder who outcrosses heavily gives you a bag of unknowns not disimilar from petshop / feeder breeders
Selection - probably the most improtant for me, how does your breeder select there breeding rats, this isnt just them saying that they select for temprement and health (most breeders say this) but how. Do they keep records, how many generations do they look back on, how do they keep in contact with there pet homes to get a full lot of data. Rats who are selected based on only 1 or 2 generations and maybe only just the parents and grandparents arent going to be as well understood and selected as rats based on many generations. Rats with no selection carry the biggest risk of anything happening, they could be super healthy or constantly sick.
Diet - i care about this one quite a bit, a rat with good nutrition from birth (and even pre birth) has a head start in terms of health, look for age appropriate diets, higher protien for babies, lower for adults. The higher quality it is generally the better, though some people work magic with kitchen scraps and a basic diet. A varied diet from a young age will increase the chance of the babies trying and trusting new foods, those raised on a lab blocks only will often distrust new foods and can be hard work getting to change diet.
Cages - rats raised in a good sized cage with lots to do will be fit active young babies who are much more able to navigate your cage at home and generally be healthier. Thise given a wheel from earlier on are more likley to wheel run as adulst, and active rats do on average live longer. Rats raised in tanks from kittenhood may struggle with climbing initiatlly, though they will learn over time, they may also have resp issues if the tank wasnt well cleaned out with appropriate substrate. They will also be less used to facing the would as a tank is more protected.
Handling - very important, a breeder who handles there babies daily from a young age will have babies who are much happier initially with human touch and need less work to socialise them. If the babies are exposed to lots of new people, things, noises etc they will settle faster in there new homes. This doesnt mean other babies wont, jsut that it will take longer and more work on your part.
Substrate - babies raised on low quality soft wood shavings are more likley to have issues later on, especially if they'e not been bred for health. Some soft wood shavings arent as bad as others though so its not a simple blanket rule.
Culling - this means different things to different people. Essentially its removing rats from a gene pool which can mean not breeding from them and keeping them as pets only but in some cases it means euthanising surplus rats, either babies or adults. This reduces the amount of data you have to call on for future generations but increases the amount of nutrients the other babies get and how well they grow.
Homing age - some breeders home from when the babies can be weaned (3-4 weeks), others from when you split the sexes (4-5 weeks) and others from whne the babies seem ready to face there new world (6-8 weeks). I personally would only get rats from someone in the 6-8 weeks bracket, not just because its better for the babies to be in a stable enviroment until they get a bit of maturity but also because the babies tend to grow up less needy of you and more comfortable in themselves. They also tend to settle faster and are less nervous though its still a big change. In the UK its considered pretty awful for anyone to home out rats younger than 6 weeks yet in the US its common practice, so theres cultural things going on here too.
Homing policy - some breeders have questionairs and a process they take to get to know there future rat owners, some just add them to a list or let anyone buy rats off them. A breeder who asks lots of questions generally cares more about where the babies go and is also more likley to stay in touch and help you with the care of the babies going forward. A breeder who doesnt is seeing the sale as a transaction which is over once you have the rat and is less likley to give support. To me supporting pet homes and having a good relationship with them is really important, it gives oyu more info to work with and gives you new friends and a knowledge your babies are well looked after