Yes, it's probably because you were looking at seats with good test results. There are some good belt fitted ones, but not up to 12 years, only up to age 7 and only rear facing. And they aren't often tested by ADAC and they aren't sold on the websites that you mention. The belt fitted ones which go up to 12 years don't get a "good" rating. Actually, the isofix ones which go up to age 12 only rarely get a "good" rating.
How old is your baby now? If they are only 9 months old then you need a rear facing seat. Most of the seats from 9 months - 12 years are only forward facing and that means that these days they are only advised from 15 months minimum (and you should rear face for longer if you can.)
You can get belt fitted, rear facing seats from 0-4 years, 9 months to 7 years (only rear facing), birth to 7 years (rear facing to age 4, then a booster) or birth to 12 years (rear facing to age 4, then a booster).
What I would recommend is either:
Get a good quality, Swedish plus tested (this is even higher safety than a "good" ADAC test) rear facing seat for age 9 months to 7 years, such as Britax Max Way or Axkid Move or Axkid Minikid. You can install this in your main car, get a second set of tethers to leave in the second car enabling an easier transfer. However you'd need to buy from a rear facing specialist.
Or buy an isofix seat (up to age 4) for your main car. The seats which offer both rear facing and booster stage up to 12 are usually a false economy, but if you are really desperate to have both features the Silver Cross Motion All Size did just about scrape a "good" safety rating (only "satisfactory" overall). However, there are lots of seats, including on sale on the sites you mentioned, offering rear facing and forward facing up to age 4 using spin and isofix which get a "good" rating, and I'd recommend these in general, with the exception being if your baby is very big for their age, then they would be a bad idea (and you should stick with the seats for "up to 7" as these are based on average children - think of this being up to age 7 clothing size.)
Then, get a cheaper belt fitted rear facing seat for the second car, such as Graco Extend R129 or Graco Slimfit R129 (which is similar to the Joie Every Stage R129). These aren't quite as good in safety as seats which have a support leg, but they are still safer than forward facing. However they are a bit annoying to use as main seats, as they have the seatbelt in the way of getting the child in when belt fitted, which is why I wouldn't recommend them as a main seat for most people.