If your going to keep and listen to them then I think $5 is a good price given that you don't even know the condition of the LPs. If you're planning on selling the rare stuff and your conscience is bothering you, cut him in for a percentage of the sale. If you call him up afterward and say, "Hey, I sold some of that vinyl for more than I paid you and wanted you to have some of the proceeds," he'll be amazed and the next time you need electrical work, you'll probably be amazed.
Dick
Dick