Is an agreement arrived at through coercion valid? The gemara in Baba Basra has a rule that “talu’hu v’zavin – zvini zevini,” that if a person is coerced into making a sale, the sale is considered valid. The reason people sell things is because they need cash – every sale is to some degree something the seller would prefer not to do, but which he must do. Coercion is just an explicit form of pressure instead of the usual implicit form.The Ba’al haIttur, however, holds that while someone can be forced to sell, he/she cannot be forced to buy (see Rama C.M. 205:12). Bnei Yisrael were in effect buyers of the Torah, not sellers. They were being forced to agree to accept something, not to give something away. So what good was their coerced agreement?
Lots of answers, but I won't spoil the fun of working it out yourself.