It all depends what kind of mower it is. If it is a cheap $100 walmart piece of junk, I would keep it for scrap metal until you get a bunch to take in. I would look mostly for mulchers and baggers and rear high wheels that are in descent shape and almost for nothing. Way back when I was doing that, thats what I would look for. Sometimes you get a mower with a bad deck and a good engine and handle bars or vice versa. Well now you have extra for other mowers as well to assemble. At first you wont see it then as your small inventory builds, but then you will have parts to move around to other mowers.
But if you cant change oil, plug, filter, blade sharpening and a good cleaning for under 20 or so dollars, your not looking in the right place for parts. First, quit going to the store to get parts, they already have there parts marked up. Also, when you start to see common parts you can sometime order 2 or 3 of each item then you dont have to keep ordering all the time. You think push mowers are expensive, wait until you get into riding mowers. So if you get a free mower and you do this, now you can sell it for 35 or 40 bucks. If you pay $10 or $15 for it now you can sell it for $50 or $60. But it also depends what brand of mower it is to. A poulan push mower, high wheel, bagger, mulcher brand new could fetch $200 to $270 maybe more? Service it and resell for $80 - $100, a little scuffed paint or something that doesnt look right, minus that from price, $65- $80. A 8 year old John Deere push with a bagger, self propelled and brand new it was a $500 push mower, service it make sure everything is ok and you could still turn it around for $150 - $200.
This is why I was telling you, if your going to do it, you have to do it right or dont do it at all. Build a good reputation and people will come around. To many factors that can go wrong fast and your stuck with a garage full of mowers.
Personally, since your not a dealer doing warranty work, you could easily use after market parts which are way cheaper then OEM parts. I still use
http://tewarehouse.com/ for OEM and after market parts. Which there not to bad, no minimum order, free shipping on orders over $200 and shipping is only $8.95 otherwise. The key here is to order for a few mowers at one time. Also need to get yourself set up with service manuals, ipl's and a few places online to order parts so on and so forth so you can easily look up parts when needed.
I also figured out then that if I needed oil or carb choke cleaner I would go to the store and buy in bulk.