Two things here that we do that might translate well here.
First: allowing a child to pick out an item is in and of itself a benefit to them. If we're at the store buying a tub of ice cream, and just one tub, if my oldest gets to decide what flavor we buy, he's happy - even though his younger sibling gets to eat some too.
A more concrete example: We fold laundry together while watching something on the television (a major reward, as they rarely watch television - laundry TV is nearly all of their TV time, ever). If one of the children sorts the laundry, that child gets to decide which show they watch; if an adult sorts the laundry, then either they rotate or we choose (if we have something we want to share with them).
Second, having a short period of timed exclusivity can work here. For example, we have three methods of obtaining items: purchasing with allowance, gaining as a reward or personal gift (think: chrismtas/birthday), and simply a purchase at our discretion. If an item is purchased with our discretion, then it's immediately a shared item: everybody can use it at any time.
If an item is given as a reward or gift, it has a limited period of exclusivity: if I buy a car and give it to my younger son, he can play with it himself for a week. After that week, it's in the shared toy bin - or if it's ever found on the ground in the common area, not put away properly in his drawers.
If an item is purchased with allowance, then they "own" it - with the same caveat that if it's left in the common area, we assume they have relinquished control over it. An incentive to be neat alongside basically any item becoming shared eventually. (In the rare case where they leave it accidentally and have strong feelings about the item, we do of course relent, but only in extreme cases.)