I concur with thoughts here. Etching from bird droppings will likely not come out without a multi-stage compound/polish. Wash and vaccuum the car and trade it in after dark (easier in the winter) and on a cloudy, rainy day. Also check Carvana. It gives you a better idea of what it may be worth in the inflated market we are in right now. For example, my GLI is worth $23,200 on Carvana and I paid $20,700 for it new a year and 8500 miles ago.
If you want, swing by my place and I have a few products we can try on the spots, if there are really only a few spots. If you are set on getting a full exterior detail, I have a guy. He worked on Manny and Tim's cars and I believe they were happy for the relatively low cost.
EDIT: Buy this and soak everything. It will not hurt paint or clear coat and may soften the stuff up enough to remove it safely.