Are you aware that when you sell a damaged ship you don't get hurt in the selling price because the ship is hurt. Eg., I sold an Orion Marauder that needed 38K in repairs for 35k. this was a 1.5k loss over the buying price. However, it was a substantial gain when you consider the damage.

Maybe make the sell price to be the stations asking price less the repair cost or make it so only ships without damage can be sold.

Or, do nothing and make it an easy valuable way to repair a ship: sell it and buy a new one.