If there are outstanding offers, users can be enabled to “accept” offers, selling their collectible(s) to the user who made the offer. You can read the outstanding offers using ListCollectibleOffersWithHighestPricedOfferFirst: 
IMarketplaceReader reader = new MarketplaceReader(chain);
ListCollectibleListingsReturn collectiblesResponse = await reader.ListCollectibleOffersWithHighestPricedOfferFirst(collectibleContractAddress, optionalCollectiblesFilter);
or, if you want to get all the sellable offers for your user, you can use the ListAllSellableOffers helper method (or just provide the CollectiblesFilter below): 
IMarketplaceReader reader = new MarketplaceReader(chain);
CollectibleOrder[] sellableOffers = await reader.ListAllSellableOffers(userAddress, collectionContractAddress, optionalCollectiblesFilter);
This will append the user’s address (seller’s address) to the inAccounts and ordersNotCreatedBy filters in the CollectiblesFilter object. This method will also handle pagination for you - continually making requests until there are no more pages - so use it with caution if you expect a large number of offers. If you do expect a large number of offers, simply use ListCollectibleOffersWithHighestPricedOfferFirst and provide the described CollectiblesFilter. Once the user has selected the offer they wish to accept and the amount of collectible instances they want to sell, you can fill the offer/sell the collectible(s): 
ICheckout checkout = new Checkout(_wallet, _chain);
Step[] steps = await checkout.GenerateSellTransaction(selectedOrders, amount);
TransactionReturn result = await steps.SubmitAsTransactions(_wallet, _chain);
if (result is SuccessfulTransactionReturn successTransaction) {
    // Handle success case
}else {
    // Handle fail case
}
You can jumpstart the process by using our Boilerplate Factory 
            BoilerplateFactory.OpenSellOfferPanel(Transform parent, ICheckout checkout, CollectibleOrder item, Action onClose = null)