I think a lot of though has gone into the user-friendly interface with easy select menus etc. I have bought / sold items easily and I find it better to put a contact number if selling so you get an idea who is buying and if they seem keen and not a time waster!
Individual experience may vary and if you can't shift the stuff your selling it's probably not GT at fault, people may not actually want to buy what your selling or your advert may not be worded well.
The only thing I can fault is I found it recently very slow to up-load photos when selling an item and I kept being timed out in the process. Although this maybe because the pic resolution was too high? Still being timed out after around 2/3 min was a tad excessive I think?
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