Finding description and impact

Incorrect validation of the listing type allows bypassing the enforcement of _minPurchaseAmt being within the range of 0 to _amount.

Proof of Concept

A listing can have two types: Single or Partial:

    enum ListingType {  
        PARTIAL,  
        SINGLE  
    }  

https://github.com/code-423n4/2024-12-secondswap/blob/main/contracts/SecondSwap_Marketplace.sol#L37

For Partial listings, _minPurchaseAmt must be set to ensure buyers cannot purchase less than the specified minimum amount.

However, during the listing of a vesting, _minPurchaseAmt is not validated correctly. Specifically, the following line is implemented improperly:

        require(  
            _listingType != ListingType.SINGLE || (_minPurchaseAmt > 0 && _minPurchaseAmt <= _amount),  
            "SS_Marketplace: Minimum Purchase Amount cannot be more than listing amount"  
        );  

https://github.com/code-423n4/2024-12-secondswap/blob/main/contracts/SecondSwap_Marketplace.sol#L253

This implementation mistakenly enforces that for Single listings, _minPurchaseAmt must fall within 0 and _amount. Instead, it should validate _minPurchaseAmt for Partial listings. The corrected implementation is as follows:

        require(  
            _listingType == ListingType.SINGLE || (_minPurchaseAmt > 0 && _minPurchaseAmt <= _amount),  
            "SS_Marketplace: Minimum Purchase Amount cannot be more than listing amount"  
        );  

With this change, the check ensures that _minPurchaseAmt falls within 0 and _amount for Partial listings, as intended.

Recommended mitigation steps

The validation logic should be updated as follows:

        require(  
-           _listingType != ListingType.SINGLE || (_minPurchaseAmt > 0 && _minPurchaseAmt <= _amount),  
+           _listingType == ListingType.SINGLE || (_minPurchaseAmt > 0 && _minPurchaseAmt <= _amount),  
            "SS_Marketplace: Minimum Purchase Amount cannot be more than listing amount"  
        );  

https://github.com/code-423n4/2024-12-secondswap/blob/main/contracts/SecondSwap_Marketplace.sol#L253