You can sell this working or not because there is a lot of lawnmower collectors out there. They have special forums that they meet to talk about, help each other, and trade or buy mowers on. If it were me I'd clean this up some and check it out to see if it still runs. It just looks very dirty from use and some gas should cut through the oil and dirt on the outside of the mower. You may need to get some new spark plugs and flush out the tank to get it working again. If you can get it working a collector will pay a bit more for this. On average, the push mowers sell for $60-75 and the gas ones sell for double this price if they are working. I would suggest that you join a lawnmower forum because they are the best places to talk to people about this mower and find a buyer for it.
Vintage reel lawn mowers sometimes have a value to collectors. This is a page about determining the value of old reel mowers.
The value of vintage lawn mowers is dependent on a variety of factors, including the condition, rarity, and age. There are forums focused on vintage mowers that might provide some help. This page also offers some estimates for some of the mowers included.
The value of vintage lawn mowers, such as this Eaton Viking riding mower, is difficult to determine. There are a number of factors to consider, including the condition, rarity, local interest, and more. Do some internet research and check for forums on the topic.
Finding information on an older electric lawnmower may a challenge. This is a page about value of a Scott's electric lawn mower.
Vintage lawn mowers sometimes increase in value over time, depending on their age and condition. This page has information about the value of a vintage Ransomes lawn mower.
Some older lawn mowers are highly collectible and may be valuable depending on their rarity and condition. Here are some ideas about finding the value of an Eclipse riding mower.
This is a page about finding the value of antique lawn mowers. There seems to be a collector for almost every type of antique or vintage tool.