It was a really bad decision to sell Eriksen I think. Maybe his performance in the last season was not good but the older statistics of him was quite good. He was really helpful for the team. Now he is at Inter but he hasn't even scored a goal or made an assist in the Serie A so far in this season.
Levy (Totteham boss) did not have too much choice as Eriksen will free in the next season if they don't sell him to Inter Milan or any other clubs. After a big successful season in a Champions League, internal problems occured in Tottenham and Eriksen did not feel happy in the club.
Back in that day, there are rumors that Real Madrid is one of big clubs want to buy Eriksen. At the end of that drama, only Inter Milan decided to buy him. By now, that is a bad decision for Inter Milan too.
I can not say that transfer is bad for Tottenham as same as what happened in Liverpool with Coutinho. It can be best for those players if they don't move. After transfers, if they come back, they won't get success most of times, Kaka and AC Milan is very good example.
Some bad transfers for great players are: Torres from Liverpool to Chelsea, Shevchenko from AC Milan to Chelsea, Griezman from Atletico to Barcelona.