The eBay copies will be fibre glass replicas. The fitment will not be anywhere near as good as the standard bumpers, so your bodyshop will have to adjust them, cuts bits off, add bits on and generally play around to get it to fit correctly. How much adjustment it needs you wont know until you get it. Some might go straight on, others may be miles out and even after fitting will leave big panel gaps.
Also, because it's fibre glass, if you hit it too hard it will crack or shatter. They don't flex much so don't take bumps like the standard ones do. Any bumps and scrapes will be fairly obvious and tricky to repair.
By the time you have paid your bodyshop to make all the adjustments, and have then been out and bought the grills and any other fitting bits you need, you may as well have spent the extra and bought the real deal.
Either buy new ones from Seat, or find a scrapped LCR being stripped for parts and get the bits you need from that.