I have a six year old child car seat that is no longer required. It has not been subject to any accident or stress. Is there any restriction on selling it or giving it away .
On parenting websites, you are always advised NOT to buy second hand car seats. If it's a booster seat for a six year old it probably isn't going to fetch much as you can get them cheaply in lots of shops (even in Aldi occasionally). If it is a six years old baby/toddler seat then it has probably been used quite a bit so may be past it's best (we had a hand me down one from one of our children and the seat belt bit on one side was really worn so we couldn't use it).