Out of the players rumored here or elsewhere Duncan Robinson and Evan Fournier are both negative players and negative trade assets. Heat and Knicks need to pay to get rid of those contracts.
Ben Simmons is a positive player but also a very negative trade asset. The Nets dont need to dump him as he provides on-court value (9.7/8.6/8.5 on per36 with excellent defense) that is worth his minutes (27 per game). He handicaps them cap-wise though, but there is not sense in moving him as his contract is so expensive you would have to pay a lot to clear roughly 18M of bad money from the cap (Simmons is 36M, but a guy producing Ben's impact in 27 MPG should be like 18M guy.. so only half of Simmons salary is overpay).
John Collins instead is a positive player and a positive trade asset despite his contract, but since he is not producing up to his contract they have to sell him on a discount. But he still nets them assets in return, unlike the other guys here.