I don't think it will make a huge amount of difference to the price you'll get for the travel system. So I'd just plan for either selling the buggy and you can throw the car seat in for the buyer as an optional extra, or keep it as a spare seat if the buyer doesn't want it, bearing in mind it won't be compatible with other prams as it was only that one it worked with.
In terms of safety if it's brand new and boxed and never been used, then it's fine even though it's been stored for 6 years. 6 years is on the longer end, but not that unusual in terms of storage after manufacture. If it was used though, even once, and then stored, that's where you may get an issue especially if there was any milk/sweat/nappy leakage etc (maybe invisible) on the fabrics and they weren't washed. But age in itself isn't that destructive to car seats - it's wear and tear which is a problem, the fact that the history of the seat becomes less clear when it passes between multiple owners, and the fact that car seat safety and features improve over time, so a new seat compared with one which is 10+ years old is likely to have better safety and be easier to use (which improves safety). Those reasons are why American car seats have expiry dates. European ones don't. The R44 safety standard is still fine as well.
Because it was a basic seat to begin with you probably will see differences between this model and the seats on sale today, even though it's less than ten years old, but I personally wouldn't be concerned about the safety of it, and would happily use it for a few more years at least, maybe 2 babies? As long as it doesn't smell musty/mildewy.
I think the leanna poster is a bot BTW, beware of any links there.