That panel is a fire waiting to happen. There is already visible heat damage around the upper quadruple tap, that is enough evidence. The term overloaded isn’t so much the right choice of words to use, but more misuse of connections. The issue is the breaker lugs are designed only for one conductor. Having more than one means the lug cant adequately handle them all resulting in poor contact. Poor contact results in excessive heat, which can lead to fire.
A few lesser issues are no clamps and conductors not in conduit.
I can certainly understand the buyers want out. The seller can sue if he wants to, any competent electrician and home inspector would say the panel’s triple and quad taps are a fire hazard with one already in the making.
An electrician will need to have it fixed. In the least combining the taps in a j box above the panel or wire nutting the the taps together, replacing any heat damaged breakers, adding clamps and getting the wire in conduit where needed.
It may also require a sub panel or new panel. I only say because the multi taps might be from new circuits added on to the house over the years. If thats the case more breakers would be needed to prevent the additions from overloading the existing breakers. 12 spaces is not enough for most houses today. The 100 amp service however is most likely enough capacity though, just more spaces are needed for those new circuits.