public Expression<Func<Purchase, bool>> forTarget(List<string> idList)
{
    Expression<Func<Purchase, string>> fc = p => p.ClientId;
    Expression<Func<Purchase, bool>> predicate = m => false;

    return idList.Aggregate(predicate, (p, id) => p.Or(m => fc.Invoke(m) == id), p => p.Expand());
}

internal class Purchase
{
    public int Price { get; set; }
    public string Description { get; set; }
    public string ClientId { get; set; }
}

public class Client
{
    public string Id { get; set; }
}

return idList.Aggregate(predicate,
        (p, id) => p.Or(m => fc.Invoke(m) == id),
        p => p.Expand());

IEnumerable<Purchase> purchases = LoadSelectedItems();
var clientIds = purchases.Select( p => p.ClientId ).ToArray();
var results = db.Clients.Where( c => clientIds.Contains( c.Id )); // Did not work.

var results = db.Clients.Where( c => c.Id == 1 || c.Id == 2 || c.Id == 3);

// start with a predicate returning false
// this is the seed of the Aggregate method
Expression<Func<Purchase, bool>> predicate = m => false;
// Now, iterate the collection and build the full predicate
foreach( var id in idList)
{
    // Build the predicate by invoking a function which returns the client id of the
    // purchase and comparing it with the value of the current id from the idList
    predicate = predicate.Or(item => item.ClientId == id);
}