FWIW - The plating appears to have worn off the valve block, possibly the result of having a valve guard on it long enough to cause damage, and it often doesn't take long for a valve guard to ruin the finish underneath it to some degree. Some people don't mind the wear; others would want to deal with the situation. If left alone, oxidation will continue, although eventually patina will develop to slow this process. It's likely you'd have to deal with the smell of brass on your hands, and there's also the likelihood that there is a small amount of lead in the brass that can contaminate your skin. The more expensive the solution to the corrosion, the more effective the result. Replating would be ideal, but not cost-effective. Compromises include lacquering or waxing the area.
Overall, if this trumpet has good valves and it plays well, the price looks pretty fair. At this price, I wouldn't let the appearance prevent me from buying it. I'd try to negotiate for a lower price, though, just to see if it would be possible to pay less.