Anything internal depends a lot on the laptop's form factor and upgrade slots (if any). GPUs are usually soldered onto the board or included in the chipset, very few laptops actually have any internal space or slots for a video card.
Your best bet is to get a laptop that has all the bells and wistles you are looking for in the first place.
As for ebay, watch out for scams, the vast majority of laptop auctions are dubious.
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