Most of it comes down to how well it looks and how well it has been maintained. Being 10 years old is a lot of mowing seasons. I've seen 5-year-old mowers that were not worth $50 dollars.
Yes, How well it has been maintained is one of the most important factors.How it looks reflects on that. It is not the years, it is the hours of run time. That is why good tractors have hour meters. I have to say that to people all of the time. They think it is like a car that is driven every day. I had one customer tell me that he read MTD mowers only last 7 years. Which is a misguided opinion based on...... Whatever? I have seen plenty of 15 year old mowers that have a lot of life left in them(all brands). In the north were I live, the mowing season is much shorter. Also many people have smaller lawns in my area. It is not hard to keep a mower going for 25 years. I say it is not hard, so hear me out before you start yelling at me. Hard is relative. Some people think changing the oil is hard so they do it infrequently or not at all. Many see replacing a spark plug as hard. Some never read the manual. Some owners do not know enough to change an air filter. Some keep it outside year round. Following the basic maintenance and storing the mower out of the elements is key to longevity.
The other things to consider is that older mowers have a better build quality. More metal and less plastic.
also, often thicker gauge steel is used. Some of the best walk behind mower engines are not made any longer. So lots of reasons to consider an older well kept mower over a new or newer used one.
Probably the Most important factor on pricing is your regional market. What is in demand and availability. How is the local economy? If the lawns are burning up and shrinking, demand goes way down.