Try not to worry about it. Even if they do have a massively kitted out nursery, it doesn't really matter if your baby doesn't regularly stay there. They are probably just excited, my PIL did exactly the same. They have literally tonnes of Brio and Duplo, swing seats, playmats, way more than we have, but it only gets used occasionally when we visit or when they look after DD. It does have advantages as we don't need to cart loads of kit to their house. Ultimately we have DD, which is the key thing 
. PIL are absolutely lovely though, just MIL got very excited with her first grandchild and is repeating with the 2nd (SIL just had a baby). It did feel a bit like PIL were also having a baby!
However, you have the 'prize' and the 'thunder' will be all yours when you have your baby. You'll have the baby in your nursery, with your things, they'll have all the stuff they bought but no baby. I don't mean that in a nasty way to your PIL, but sometimes it's helpful to remember that when your baby's born, it will most likely matter less to you. It certainly did to me.
If they offer you stuff, don't take anything you don't want, just say you've seen the exact one you want that matches the nursery decor/fits just right in the space/recommended by your best friend.
Unfortunately you can't stop them spending their money. I found the best bet was a smile sweetly and take a deep breath. Ultimately if they're just excited then it's not worth falling out with them over it as they could be very useful babysitters in the future.