Осталось еще всего лишь сделать примерно 95% от всей работы

BusinessLogic/BusinessLogic/SellLogic.cs

@@ -0,0 +1,12 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace BusinessLogic.BusinessLogic
+{
+    internal class SellLogic
+    {
+    }
+}

Contracts/BindingModels/PurchaseBindingModel.cs

@@ -12,8 +12,8 @@ namespace Contracts.BindingModels
 	{
 		public Guid Id { get; set; }
 		public DateTime DatePurchase { get; set; }
-		public required IUser User { get; set; }
-		public required List<IProduct> Products { get; set; }
+		public Guid UserId { get; set; }
 		public PurchaseStatus Status { get; set; }
+		public Dictionary<Guid, (IProduct, int)> PurchaseProducts { get; set; } = new();
 	}
 }

Contracts/Converters/PurchaseConverter.cs

@@ -14,16 +14,16 @@ namespace Contracts.Converters
 		{
 			Id = model.Id,
 			DatePurchase = model.DatePurchase,
-			User = model.User,
-			Products = model.Products,
+			UserId = model.UserId,
+			PurchaseProducts = model.PurchaseProducts,
 		};
 
 		public static PurchaseBindingModel ToBinding(PurchaseViewModel model) => new()
 		{
 			Id = model.Id,
 			DatePurchase = model.DatePurchase,
-			User = model.User,
-			Products = model.Products,
+			UserId = model.UserId,
+            PurchaseProducts = model.PurchaseProducts,
 		};
 	}
 }

Contracts/Converters/SellConverter.cs

@@ -0,0 +1,13 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Contracts.Converters
+{
+    public class SellConverter
+    {
+
+    }
+}

Contracts/ViewModels/PurchaseViewModel.cs

@@ -12,8 +12,9 @@ namespace Contracts.ViewModels
 	{
 		public Guid Id { get; set; }
 		public DateTime DatePurchase { get; set; }
-		public required IUser User { get; set; }
-		public required List<IProduct> Products { get; set; }
+		public required Guid UserId { get; set; }
 		public PurchaseStatus Status { get; set; }
+        public Dictionary<Guid, (IProduct, int)> PurchaseProducts { get; set; } = new()
+
     }
 }

DatabaseImplement/Implements/SellStorage.cs

@@ -13,7 +13,7 @@ namespace DatabaseImplement.Implements
 	{
 		public SellBindingModel? Delete(SellSearchModel model)
 		{
-			
+            throw new NotImplementedException();
         }
 
 		public SellBindingModel? GetElement(SellSearchModel model)

DatabaseImplement/Models/Product.cs

@@ -83,7 +83,6 @@ namespace DatabaseImplement.Models
 			IsBeingSold = model.IsBeingSold;
 			Amount = model.Amount;
 		}
-
         public ProductViewModel GetViewModel
         {
             get

DatabaseImplement/Models/Purchase.cs

@@ -2,10 +2,13 @@
 using Contracts.ViewModels;
 using DataModels.Enums;
 using DataModels.Models;
+using Microsoft.EntityFrameworkCore.Metadata.Internal;
 using System;
 using System.Collections.Generic;
 using System.ComponentModel.DataAnnotations;
+using System.ComponentModel.DataAnnotations.Schema;
 using System.Linq;
+using System.Runtime.Serialization;
 using System.Text;
 using System.Threading.Tasks;
 
@@ -16,24 +19,62 @@ namespace DatabaseImplement.Models
 		public Guid Id { get; set; }
 		[Required]
 		public DateTime DatePurchase { get; set; }
+        [Required]
+		public Guid UserId { get; set; }
         [Required]
 		public PurchaseStatus Status { get; private set; } = PurchaseStatus.Unknown;
+        private Dictionary<Guid, (IProduct, int)>? _purchaseProducts = null;
+        public virtual User? User { get; set; }
+		[DataMember]
+        [NotMapped]
+        public Dictionary<Guid, (IProduct, int)> PurchaseProducts
+        {
+            get
+            {
+                if (_purchaseProducts == null)
+                {
+                    _purchaseProducts = Products.ToDictionary(e => e.ProductId, e => (e.Product as IProduct, e.Count));
+                }
+                return _purchaseProducts;
+            }
+            set { }
+        }
+		[ForeignKey("PurchaseId")]
+		public virtual List<PurchaseProducts> Products { get; set; } = new();
         public PurchaseBindingModel GetBindingModel() => new()
 		{
 			Id = Id,
-			DatePurchase = DatePurchase
+			DatePurchase = DatePurchase,
+			UserId = UserId,
+			PurchaseProducts = PurchaseProducts,
+			Status = Status
+		};
+
+        public PurchaseViewModel GetViewModel() => new()
+        {
+            Id = Id,
+            DatePurchase = DatePurchase,
+            UserId = UserId,
+            PurchaseProducts = PurchaseProducts,
+            Status = Status
         };
 
         public static Purchase ToPurchaseFromView(PurchaseViewModel model, Purchase purchase) => new()
 		{
 			Id = model.Id,
-			DatePurchase = model.DatePurchase
+			DatePurchase = model.DatePurchase,
+            UserId = model.UserId,
+            PurchaseProducts = model.PurchaseProducts,
+            Status = model.Status
         };
 
 		public static Purchase ToPurchaseFromBinding(PurchaseBindingModel model, Purchase purchase) => new()
 		{
 			Id = model.Id,
 			DatePurchase = model.DatePurchase,
+            UserId = model.UserId,
+            PurchaseProducts = model.PurchaseProducts,
+            Status = model.Status
         };
 
 		public void Update(PurchaseBindingModel model, Purchase purchase)

DatabaseImplement/Models/PurchaseProducts.cs

@@ -0,0 +1,23 @@
+using System;
+using System.Collections.Generic;
+using System.ComponentModel.DataAnnotations;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace DatabaseImplement.Models
+{
+    public class PurchaseProducts
+    {
+        public Guid Guid { get; set; }
+        [Required]
+        public Guid PurchaseId { get; set; }
+        [Required]
+        public Guid ProductId { get; set; }
+        [Required]
+        public int Count { get; set; }
+        public virtual Product Product { get; set; } = new();
+        public virtual Purchase Purchase { get; set; } = new();
+
+    }
+}

WebApp/Pages/Cart.cshtml

@@ -0,0 +1,4 @@
+@page
+@model WebApp.Pages.CartModel
+@{
+}

WebApp/Pages/Cart.cshtml.cs

@@ -0,0 +1,13 @@
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.AspNetCore.Mvc.RazorPages;
+
+namespace WebApp.Pages
+{
+    public class CartModel : PageModel
+    {
+        public void OnGet()
+        {
+
+        }
+    }
+}

WebApp/Pages/Catalog.cshtml

@@ -0,0 +1,4 @@
+@page
+@model WebApp.Pages.CatalogModel
+@{
+}

WebApp/Pages/Catalog.cshtml.cs

@@ -0,0 +1,12 @@
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.AspNetCore.Mvc.RazorPages;
+
+namespace WebApp.Pages
+{
+    public class CatalogModel : PageModel
+    {
+        public void OnGet()
+        {
+        }
+    }
+}