[Uniques v2] Buy offers with the off-chain signatures

[jsidorenko]

Idea:

If we take the classic NFTs auction model, we would see a lot of buy offers being sent on-chain thus polluting the blockchain space and congesting the network. There is a way to achieve the same level of functionality by utilising the off-chain signatures.

For example, Alice decides to sell her NFT on the marketplace X. Bob wants to buy that NFT, pre-signs his offer and sends that signature (with offer details) in any way he wants (e.g. to the marketplace, or via system remarks).
Charlie also wants to buy that NFT for a higher price, so he submits his off-chain signature as well.
Now, Alice could choose the best offer (signature) and submit it on-chain.

On a technical level, the offer's object could include an asset the buyer is willing to pay with and an array of tips. An optional block number could be set as a deadline to control the validity of the offer.

Possible issue:
An off-chain signature doesn't reserve the currency on the user's account and thus can't guarantee the offer will be valid by the time of execution. As a possible solution, we can add the nonce param to the signed object meaning the offer will become invalid if the buyer decides to execute another transaction meanwhile. With that solution, we're offloading the check of the user's balance to the UI.

Technical implementation:

pub struct BuyOffer<CollectionId, ItemId, MultiAssetBalance, BlockNumber, AccountId, Tips> {
	/// Collection id.
	pub collection_id: CollectionId,
	/// An item id to buy.
	pub item_id: ItemId,
	/// A price the buyer offers.
	pub bid_price: MultiAssetBalance,
	/// A block number this is offer is valid until
	pub deadline: Option<BlockNumber>,
	/// Item's owner, will be credited with `bid_price`.
	pub item_owner: AccountId,
	/// Off-chain buyer to debit.
	pub signer: MultiSigner,
	/// An account that will receive an item.
	pub receiver: AccountId,
	/// An array of tips
	pub tips: Tips,
}

impl<CollectionId, ItemId, MultiAssetBalance, BlockNumber, AccountId, Tips>
	BuyOffer<CollectionId, ItemId, MultiAssetBalance, BlockNumber, AccountId, Tips>
where
	BuyOffer<CollectionId, ItemId, MultiAssetBalance, BlockNumber, AccountId, Tips>: Encode,
{
	/// Returns whether `signature` is a valid signature for this Offer
	/// and was created by the signer.
	pub fn verify(&self, signature: &MultiSignature) -> bool {
		let data = Encode::encode(&self);
		signature.verify(&*data, &self.signer.clone().into_account())
	}
}

// ....

pub fn accept_buy_offer(
	origin: OriginFor<T>,
	offer_details: BuyOfferOf<T>,
	offer_signature: MultiSignature,
) -> DispatchResult {
	let sender = ensure_signed(origin)?;
	ensure!(offer_details.verify(&offer_signature), Error::<T>::InvalidSignature);
	// ...
}

[jsidorenko]

As an extra idea - we could extend the Offer's object by adding an optional buyer's NFT that he offers. That would allow exchanging NFTs in an atomic way.