(Heavily inspired by Roblox Deathrun’s shop.)
(I have updated it, any new opinions on it?)
I think it looks nice, though having the location/look of the item’s name/price might be good to help us help you a bit more.
Looks really nice.
Only thing I’d add to it is the close button though.
I’ve thought of just having a button on the side that you can click to open and close 
I love it, the buttons look amazing with the 3D aspect and the only thing I think would improve it is having a close shop button as Legoracer said.
I prefer having close button in shops when playing games, although this is fully subjective and depends from person to person.
Maybe a good in-between could be a bound key/button to fast toggle the shop?
If you gave me both button on side and the keybind for toggling, I’d probably look for close button. 
Should I try using viewport frames or pictures custom made?
Viewport frames are not released outside of studio yet.
If you’re planning to release the game relatively soon (this month?), I’d use custom images.
ViewportFrames are really promising (and would take less work to add new items to the shop). The only caveat is what @Legoracer said - they won’t be out for a bit.
If you’re fine with drawing/making an icon for each item out of studio, then make them - especially if you want to stylize them. Otherwise I’d highly recommend ViewportFrames.
I love the design but I’d recommend rounding the image labels and the details part.
This happens with me as well. I just resize using the properties window when this happens, no clue how it’s fixed.
Looks fairly solid to me. I would spend some more time on choosing the right margins and padding for all elements to make it seem a lot more visually consistent. For example, your scrollbar is hugging the side of the right frame and your “item details” panel has smaller borders than your “item thumbnails” frame.
Maybe you should change the color on the top from blue to white and these buttons “Trails” , “Emotes” make blue so whole inventory would be white, but I am not really sure just suggesting.
Looks very nice, something I’d recommend doing though is getting gaps between frames equal so it looks cleaner. Right now the gaps between the frames are inconsistent.
Along with that you have both square frames and rounded frames, which is inconsistent as well.
Looks hella good! Nice 
Looks good, but I do have a few ideas!
Perhaps make a border for the scrolling frame, that way the scroll bar isn’t touching the border of the frame. The shadow under the buttons on top are a little bit to far down, and look like they need a taddd bit of transparency, other than that: Looks great!
If you guy’s didn’t notice, the shop GUI has been updated, any new opinions?
