At my table, when items are first acquired the party collectively decides who can put the item to best immediate use. If/when there is a chance to sell the item, and the party wants to sell, any PC can opt to pay that price to buy the item from the party. If multiple PCs want it, it goes to the highest bidder. A PC who wants a particular item before the party tries to sell it can always make an offer sooner--it's up to the party if they want to accept.
Whether or not there is an opportunity to sell the item varies a lot from campaign to campaign. There is a major metropolis that has a Great Market where just about anything can be bought or sold, but it can take weeks and there are multiple levels of middlemen and agents involved (all of whom want a percentage or an upfront fee). Still, the very existence of that market helps set prices for unique goods throughout the continent, as skilled appraisers with good connections will likely be able to give reliable ballpark estimates from what any given item would sell for at the Great Market. But some of my campaigns have been set far from enough from civilization that opportunities to sell are fewer.
Whether or not there is an opportunity to sell the item varies a lot from campaign to campaign. There is a major metropolis that has a Great Market where just about anything can be bought or sold, but it can take weeks and there are multiple levels of middlemen and agents involved (all of whom want a percentage or an upfront fee). Still, the very existence of that market helps set prices for unique goods throughout the continent, as skilled appraisers with good connections will likely be able to give reliable ballpark estimates from what any given item would sell for at the Great Market. But some of my campaigns have been set far from enough from civilization that opportunities to sell are fewer.