Depending on what you pay for them, there is only one question.... do they hold pressure?
Not quite the full story, now the OP'er has new kegs (what is the name stamped into them?) we can go into this subject into a little more depth for others thinking about kegging!
Just because a keg won't hold pressure doesn't mean it's no good. What if it has an old faulty rubber seal to start with? Is Jo Blow going to take some CO2 and a pressure gauge to do a leakage test on kegs for sale? Has the seller done a leakage test with a gauge? What if the seals are all new and it holds pressure, however the corrosion from salt, a sodium based cleaner or chloride from tap water sitting inside is only 2 microns away from creating a hole in the keg? Disaster, (sticky tape won't help you as I have seen attempts in the past to plug a leaky keg full of raspberry syrup fail before)!
Kegs should be taken apart completely (including PRV and tank plugs) and inspected for any signs of corrosion (in SS, corrosion does not leave a stain like mild steel it "pits", like moon craters). It doesn't matter what the original seal are like, because your going to replace them anyway!
Corrosion and new seals do not make a good buy either!
Next we will get onto the ownership issue, especially if we are talking about certain 50L (or other size) kegs, owned by breweries or liquid product companies with the police and their unlimited budget when it comes to prosecuting people when they think they have a good case. Beware branded kegs, unless there is a certificate for the "brand" name company on the keg stating a sale of such keg and including the stamped serial number.
Then we will need to talk about those tank plugs. They are the things that screw into or onto welded fitting on the kegs for gas or liquid. If you get the Ball Lock type, you are better off, as there is far greater proliferation of this type of fitting. In the early days of kegging home brew, the home brew "movement" originally had the option of Pin or Ball Lock. Because it was too confusing to go to the superior Pin lock system and Coke corporation had an unlimited budget with which to pursue keg thieves with, so they went for the Pepsi Ball lock choice. If you get a keg with Pin Locks, you will find only high-quality disconnects are available as opposed to the ball lock stuff which every "reputable Aussie" importer of quality Chinese plastic crap is selling on eBay for less than 1/2 the price. The balls fall out of the fittings, etc.
I have no affiliation with any retailers at all! Just my 2c worth. (only had 2 standard drinks after a long night shift)