Very well known scam. Just ignore and block. They won't do anything — they try these kind of scams on huge numbers of people, and they won't waste any more time on someone who hasn't fallen for it. Unfortunately lots will fall for it, and they'll just move onto the next.
The email from "PayPal" is not actually from PayPal — it'll be a fake email set up to look like it's from PayPal, and if you check the sender's email address you'll find it's not legit.
The other week I sold a high-value piece of furniture on Gumtree, and thankfully the buyer turned out to be totally legit, but I was on very high alert the whole time. I even called up Gumtree's customer service at one point to ask if they were able to let me know if the buyer had any dodgy history on the site (they couldn't tell me anything due to privacy), but the person I spoke to did give me a run-down of all the most common scams, and the sequence of events you've described was one of them, almost word for word (including the wheelchair thing, the courier payment via Amazon, the fake pending PayPal payment email etc.).
As a seller, the only options I'm willing to offer to buyers on Gumtree are (a) cash on collection, for relatively small-value items, or (b) bank transfer if the items are more expensive, and the transfer needs to have cleared before I hand over the item. For large-value items, there's an increased risk of fraud via fake bank notes (and fake £50s are likely to be more common than, say, fake £5s), which is why I prefer bank transfer for more expensive items.
Note that I, as a buyer, would never actually go for option (b) when dealing with a seller on one of these marketplaces, as there is no buyer protection whatsoever. However, as a seller it's probably the safest method, so long as you check with your bank that the payment truly has cleared, as there's no way of reversing a bank transfer payment (whereas a PayPal payment can be reversed under some circumstances). Always prioritise minimising risk to yourself, and decline the sale if you sense any funny business whatsoever.