Hi passionate4nature,
Latex Green is a Dunlop latex manufacturer which is based in Sri Lanka as you mentioned. They make high quality 100% natural Dunlop latex which is used by many manufacturers in their latex mattresses including many of the "choose your own layer" Dunlop mattresses and many other high quality mattress manufacturers as well. I should also mention that there was no "organic" latex 2.5 years ago.
As far as what to look for ... I personally wouldn't buy a used mattress from someone I didn't know for aesthetic, hygienic, and health reasons but if I were in that hypothetical situation I suppose ... I would look for the overall condition of the mattress and any obvious signs of wear or mistreatment, rips, or other damage, the cover condition and any wear spots or thinning in the cover, any signs of staining or attempts at stain removal in the cover on both sides (slight water stains). I would open any zip cover and fold back each layer on both ends and check for the same things on both sides of each layer. I would also check for any signs of bedbugs (small dark blood stains on the mattress) or signs of mold or mildew.
I would also ask why they were selling it and see if their reason made sense to me and "fit the evidence" in my best judgement. I would ask if they had their original receipt and whether they were the original owner. I would check the law tag to see if the manufacturing date was close to when they said they purchased it. I would check the surroundings and their own cleanliness to see how clean they were in their living habits. I would ask if they had always used a mattress protector and what type (and hopefully have them show you).
Finally I would test the mattress for PPP (Pressure relief, Posture and alignment, and Personal preferences) as objectively and carefully as I could in the same way I would any new mattress to make sure it fit my needs and preferences (although if it didn't then re-arranging the layers or the purchase of a single layer may correct this if you are willing to take on the extra expense). I would also check for any soft spots or impressions (virtual or visible) ... especially if they seemed heavier and the latex was soft.
If all of this looked good and you were completely comfortable with the purchase, the quality of the materials, the lack of any warranty, and the price was right ... then I would consider it ... hypothetically
Phoenix