I use ML mostly to find out the prevailing local price of sometime before going to a store. Especially with electronics, prices in local stores seem to be all over the lot, and I find many vendors make up a price that's almost always higher than what is advertised on ML. When I check prices on ML and then go to a local store and ask the price, and they tell me $150, I reply that everyone is selling the same thing on ML for $95-100, and I usually get that price.
I also occasionally use ML to see if something I haven't found in the neighborhood is in the market at all and, if it is, to locate a vendor for it. Most of the ML vendors with physical stores, which seems to be a majority of the listings, show their logo, which you can google, or enough hints about their address to locate them.
When I was in the US, I used eBay all the time. Here I'm less comfortable making purchases online. When I do, I always choose cash payment so I can complete the deal face-to-face with the seller.