Man, if it's a flip just keep it all NG and save your money. Propane will not be such a deal breaker as to sink all that money converting it. If you keep it, spend the money and enjoy it. But the difference in sale price would be minimal. To restore it to NG all you need is the flex hose supplied by Weber with the grill. They are available pretty cheap since it's a safety issue. I guess squirrels and rabbits chew on them. I think the hose and fittings are like $25.00. Just take off the propane hose and screw in the NG hose. Now that's easy!!! I'm guessing that grill all cleaned up is worth $400-$500 in the right market.
Also is there any reason you can't do an NG hookup? A few pipes and fittings are not that expensive. If you live in an apartment that may not be an option but generic parts are way less expensive than name brand parts.