Submitted by fyamf, also found by 0xastronatey, 0xKann, 0xPSB, 4rk4rk, ABAIKUNANBAEV, Abdessamed, AshishLach, aster, BajagaSec, BenRai, Bloqarl, bugvorus, DanielArmstrong, Drynooo, Fitro, honey-k12, ITCruiser, itsabinashb, kimnoic, KupiaSec, lightoasis, Olami978355, oualidpro, peanuts, pontifex, pulse, queen, Sabit, Sabit, shaflow2, tusharr1411, and wickie0x
https://github.com/code-423n4/2024-12-secondswap/blob/main/contracts/SecondSwap_Marketplace.sol#L253
Incorrect validation of the listing type allows bypassing the enforcement of _minPurchaseAmt being within the range of 0 to _amount. 
A listing can have two types: Single or Partial:  
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:  
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:  
With this change, the check ensures that _minPurchaseAmt falls within 0 and _amount for Partial listings, as intended.
The validation logic should be updated as follows:  
https://github.com/code-423n4/2024-12-secondswap/blob/main/contracts/SecondSwap_Marketplace.sol#L253
calvinx (SecondSwap) commented:
