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).