You should NOT buy a used car seat, especially one from craigslist. The most important question to ask yourself when buying a used car seat is, "Do I trust this person with my child's life?" Car seats are only a 1 time use safety device. After a crash, they must be disposed of and replaced. If the person who you bought it from lied to you about the seat's condition, and if you were in a crash, the seat wouldn't perform as intended. The harness could have been worn and could break through the shell. It may have hair line cracks that are invisible to the naked eye.
In addition to this, car seats expire after 6 years from the date of manufacture (seats made out of steel, like the Diono Radians have an 8 year expiration date with the harness). Seats expire because they are exposed to UV lights, seats are left out in the sun in the car (it can easily get over 100 degrees Farenheit in a car in 70 degree weather), and the materials in the seats degrade over time. Plastics become brittle over time.
In addition to this, I wouldn't dye the infant seat cover itself; car seat covers have flame retardents, so if in a crash, it won't just go up in flames. You cannot dye the infant seat cover without consulting the manufacturer of the seat.
As a side note, you don't have to use an infant seat from birth. Instead, you can buy a convertible car seat that you can use rear facing. Convertibles can be used rear facing and after the child has outgrown rear facing in it, the seat is installed forward facing.
The only time an infant should be in a reclined position is when they are traveling in the car; infants should be lying flat on their back when in a stroller. Instead, I would buy a stroller that folds flat. Furthermore, never put an infant seat on top of a shopping cart. Thousands of babies are injured every year from falls.
Car accidents are the leading killer for children under 12 years of age in the United States.