When you buy a memorabilia, you must always set a resale price. It's mandatory.
Your memorabilia is automatically put up for sale on the market at this price and the sale will take place as soon as another user buys it at this price.
Other players may own your memorabilia, and as the cheapest units are always sold first, you must make sure to regularly consult the different selling prices on the market, to adapt your sales strategy.