I agree with vet refs -- nothing tells me quite as much about rescue adopters as going through the entire file with vet staff -- but knowing the right questions is critical. We don't have newbie volunteers do vet reference checks for a reason. It's not just for routine care (heartworm prevention, annual check ups, appropriate vaccines or titers), but also looking at the full history of accidents due to inattentiveness (ever have one hit by a car?), history of illness care (ever decline treatment or diagnostics due to cost?), history of senior care (did the past dogs get senior wellness panels, appropriate arthritis management, humane palliative care at the end, etc.?)
My experience is personal checks rarely reveal anything useful unless it's a parent saying their adult child is already over-extended and doesn't need another pet. That's really rare. It's usually some friend who says glowing things about them. Landlords can be VERY useful sometimes, esp. if they've known a tenant a long time.
Be very, very skeptical of young families with young kids. In rescue, they are the most likely to return ANY dog, but especially puppies -- the mouthing often scares small kids, and protective parents decide it's "aggression" and return the pup.
I also firmly believe in home checks. Some rescues think they're unnecessary, and I know they're not typical of breeders, who may have to sell out of state. BUT, if you can do them locally, here's what I like to do -- and it works: we're there long enough to get people to relax and open up as we walk the fence line, look for poisonous landscaping, point out toxic chemicals left out at dog-level, point out attractive things for puppies to chew (cords, fancy pillows, etc.), help think through crate placement and training, etc. We help plan the puppy-proofing of the home during the home check. Then a few days later, we deliver the dog to their home and spend at least an hour getting everyone settled, helping them practice redirecting during mouthing, explaining why it needs to be on leash in the home for the first few days, etc. Even if they've had pups before, it's a refresher.
By the end of all that, they're usually friends who will be comfortable texting if any worries come up! That's what you want -- help them out with small problems before they can become big ones that trigger a return.
As for chipping, some chip companies make it harder to transfer chips than others. Some just want to pocket the $20 transfer fee. My experience is that AVID will contact the former owner upon getting a transfer request, so if you chip them to you, they will contact you before transferring. You can probably put the pup's people as alternate contacts. Petlink (my favorite) has a chip with a guardianship feature for rescues that you might ask for by contacting them directly -- it cannot be removed. I don't know if they offer it to breeders, but it's worth asking. They might sell you a packet of 25 chips for your vet to insert (yeah, it's a lot of chips...). If the pet lands in a shelter, the holder of the guardianship is contacted along with the registered owner -- so our rescue gets notified along with the owner. I also can see on our screen when owners update info -- and we'd get notified if they tried to transfer.
Most people are too lazy to ever register microchips (something like 2/3 of them go unregistered nationwide). So if tell your buyers YOU will handle the microchip registration, they'll probably just go with it. However, if the dogs are chipped to you, be ready to get calls from Animal Control out of state if the dog gets out of its yard -- that's why I would put the buyers down as alternate contacts at least. :/
You might also download a few GS rescue applications from rescue websites, to see the kind of questions we ask about lifestyle, training history, etc. The apps are mostly designed to get people to do some self-reflection about what's realistic for themselves. It will give you some ideas of the kinds of questions you might want to ask when people call you.
I know from rescue that whenever we get puppies out of shelters, it brings out the WORST adopters. I have to become like a protective mama bear over them to get them good homes -- I think we had 30 apps for the last 2 pups we had, and the vast majority of them were not the kind of experienced, diligent inside homes I wanted for them. It took a lot of patience to find them great homes willing to put in the time, invest in training, and commit to them for life. I can only imagine how bad it is when people think that they're just items for sale, so showing up with money equals getting a pup!