I dont have a shop but I have a fairly large garage that slowly turned into something of a mechanic shop over the last years. (kind of lol) I usually find my mowers that I sell by using facebook marketplace, craigslist and offerup. Ive had pretty good luck with the mowers I buy (or free sometimes) to fix and usually cost originally from $50 -250 depending on the condition of the machine and cost about 100 or 150 dollars to fix... usually. I usually clean the carb because that is usually the problem of the mower apart from it being crazy dirty and abused. If it doesnt start after cleaning the carb, I check for other problems and if I have to, buying a new carb. They also usually need a few new tires and blades and usual services like oil change, greasing, and sometimes a little paint or glossy clear paint to make it shinier. After im done, I check it and mow the grass with it for 40 minutes or so to see that there are no problems. After this I put them up for sale and are 90% of the time sold after less than 5 days. Ive also had some occasions were the mower is sold in less than a day!
I also fix up some two cycle engines like blowers or weed eaters but I mainly fix push and self propelled mowers and riding mowers.
I price my riding mowers just in the middle and in my area the usual price is from 600 ( an average 2007 42" cut Craftsman in decent shape) to 1400 or so for a nice 2018 54 inch cut mower. For example a 2009 Troy Bilt Pony I sold was priced at 760 and I sold it at 720, it was in like new condition with 2 new tires, blades, carb, air, fuel, and gas filter, and just greased. I like for the machines I sell to be extremely clean so I like it and the person that buys it as well. Another example would be a 42" cut 2017 Cub Cadet XT1 I sold for 910 dollars. It also had new blades, cleaned carb, tires, filters etc and looked very nice and worked perfectly.
For the price of a standard push mower I sold was for 105 dollars. (nice looking murray 140 cc push mower) It had a sharpened blade and new carb, and oil. I bought it for 15 bucks when it was for fixing. Larger push mowers for example a honda 160 cc self propelled with bag, blade stop and choke control I sold for 250 which was also nice condition.
Ive only had one person contact me for a return of the mower but apparently didnt work properly when he used it but when I used it to cut the grass twice it worked just fine.
I only do this (apart from earning some money) for fun because I like fixing mowers but not much else. Im sort of a DIYer myself lol