I wouldn't say they are wildly overpaid, but I think they don't have much value as trade assets even if their contracts are close to appropriate market value.
They are just not the type of players who will entice opposing GMs to overpay, so their value in trades is mainly salary filler. They are not bad enough for a team to ask for much i the way of extra compensation for taking on their contracts, yet not good enough for most teams to be interested if they already have an adequate incumbent starter already established on their roster.