I always assume the absolute worst about used goods, as Tiare and others above have said. Bags, along with cell phones, are some of the most bacteria-ridden of daily items in modern life.
An evening news program I saw about a dozen years ago took several women's handbags to a laboratory and had them tested; the results were interpreted by public health experts to be loaded with E. coli and other potentially harmful bacteria. Most of the handbags tested contained traces of human and non-human feces.
Many women set their bags down on the floor in multiple venues--yes, including the public toilet/washroom. Of those women who hang up their bags on the hooks in Western-style public toilet stalls, the vast majority still take down the bag as they exit the stall using the hands they've just used to attend to their excretory functions. (I cover my hand with a napkin or wad of toilet paper whenever I touch my bag on the way to wash my hands and whenever I touch any public handle or surface like a cafeteria tray, a salt or pepper shaker at a restaurant, etc.--for my protection as well as that of others!)
Yet, I've bought several used bags on eBay and the like. But the key is I only buy ones I know or suspect are of a durability and materials that can hold up to the rigors of the cleaning and disinfecting process I will subject them to! Dooney and Bourke's Florentine leather, because it hasn't been shrunken/treated for water repellence and because it's very sturdily stitched and constructed, works well for my process.
My steps are roughly:
1) As soon as a used bag enters my home, I take a hairdryer on a medium-hot setting and blow warm-to-hot air into all the crevices of the bag, internally and externally. This helps flush out any crumbs/debris and, heaven forbid, bugs.
2) I spray the entire outside of the bag--again, only do this if you're sure the material can withstand it or if you haven't invested enough money to worry--including the all-important handles with a quaternary disinfectant (Lysol brand works) and allow to air-dry for 20 minutes. If the lining of the bag looks free of staining/dirt, I will spray that as well at the same time.
3) For stains and dirt in the lining, I pull inside-out the lining as much as possible, run water from the tap over it to saturate it, pour dishwashing liquid (like Dawn or Joy) onto it and rub the cloth against itself until a lather appears, and begin scrubbing the fabric vigorously with a short-bristled, firm dish or vegetable brush I reserve for cleaning the inside of bags. Then I flush the cloth with as much water as needed to remove the remains of the dirt and all of the soap's lather.
4) OPTIONAL: for bags with really ingrained-looking, visible dirt on any part of the exterior including handles, I either dunk the entire bag in a bucket or basin of soap and water, scrub with my brush, and rinse in running water, OR, yes, I have put entire bags into the washing machine through a full wash cycle including the final spin! (top-loading, center-spinning classic model). But whether by basin or by machine, the whole bag needs to be dunked; spot-washing is a big risk because it can leave darker water stains on the parts you spot-washed.
5) For leather--again, leathers I know to be flexible and receptive to all this manipulation, like Florentine/vachetta/aniline--I slather on a good coat of lightweight leather conditioner like Leather CPR to restore hydration and pliability. This works best when the leather is still damp enough to absorb the hydration.
6) Hang to drip-dry. I keep a plastic garbage bag for this purpose to spread out on the kitchen or bathroom floor underneath the portable laundry drying rack or hanger on which I hang the bag--preferably with the lining still pulled out and upside down to speed up the dripping process.
[If necessary, stuff the bag for several days in order to restore any shape it may have lost during the process.]
7) Enjoy my blissfully clean bag and do my best to ensure not coating it with potentially harmful bacteria (see public washroom habits above, etc).