New Construction - Water Stained Drywall

Hi all! I am in the process of buying the house I am currently living in. The person that I am buying it from had 3 boys, who are very sweet, but also.. are boys. So the walls in some of the rooms don't look too great. I have the colors that were originally used when the house was built 5 years ago. But can I just paint over all the scratches and scuff marks? I tried washing the walls, even used a magic eraser but that seemed to be pulling off some of the paint so I stopped that quickly. I am just worried that if I paint the bad areas, the paint won't match due to it being 5 years old. I really don't want to have to repaint all of the walls and such, especially since the appraiser will be here sometime this month. Any and all advice would be greatly appreciated!

Hi, My problem is with a fiberglass front door and I am only painting the sides and the inside since HOA is responsible for the exterior (I think, haven't check that out). I chemically stripped the old paint to the bare fiberglass, -apply mineral spirits, let dry, -scrapped/sanded dried left over CitriStrip, -washed with TSP substitute and went over twice with damp rags to rinse, -apply mineral spirits again, -sanded with 220 grit lightly, -apply with denatured alcohol or 91% rubbing alcohol, -and then primed the raised panels, and let dry overnight. I primed with a never opened old can of Zinsser 123 primer. I am in Hawaii so the temperature is in the high 70's or more and there is a nice cool breeze where I am and humidity is high. I am painting it mounted on the hinges. I was going to prime the rest of the fiberglass door. Then I decided to test and was able to scrape off some of the primer with my fingernails, but not all of the primer. The edges where the primer was thick and I may not have sanded it well. The area with a light coat of primer seems to adhere better. The primer on the trim and door jamb seems to adhere correctly. Now, it is two days since I primed and I can still scrapped off some primer with my fingernail. It seems that in Hawaii, I need to wait overnight to recoat, even though the instruction on the can say you can recoat in 1 hour. Must be the humidity. I did a google search and some forum suggest using some kind of bonding primer. I am hoping not to buy anymore primer. I am planning to sand again more thoroughly with 100 or 220 grit and then prime again. I plan to do two coats of primer and let dry overnight and then two coats of semi-gloss off white as topcoats. Please let me know if you have a better suggestion. Thank you very much for your time and help.