I need to create an in-game shop, so basically i need to select products and customize them, put them in my inventory and then buy them. any good thing i could find in the marketplace? till now no one (of those i have seen) looked good to me.
You are doing it wrong. Buying big and complicated code systems from marketplace has one huge fault. You can be almost sure that two different systems bought in marketplace will be not compatible. So even if there was perfect shop system for you, if you bought it, and then bought for eg. inventory system. You would have huge problems with connecting them together. Those big code blueprints on marketplace are meant for speeding up development, not ready to use blocks.
I strongly suggest you learn blueprints and code your first system by yourself. Also market (shop) system is bunch of other systems put together, you need npc dialogs, npc ai, player inventory, all that before you can buy or sell. And if your game is multiplayer first thing you need is multiplayer code. Changing singleplayer code to multiplayer is harder than just writing that code as multiplayer from very beginning.
PS. if you have some cash and are unwilling to learn blueprint you should hire programmer to make that code for you, else just start coding,