First off sublingual means "under the tongue" as in absorbed directly into the blood through the pores of the tongue or something like that.
I seriously doubt that colloidal silver can be absorbed this way.
Secondly, while it is possible to make extremely high PPM's of Colloidal Silver (I think someone here made 500PPM or 1000PPM once), where many people here make 320PPM, you CANNOT do this without a STRONG capping agent like gelatine and then too, the color of it would be dark brown mud.
If what these so-called sellers of 500PPM are selling isn't dark brown and opaque where on diluting a little to 20PPM it isn't the correct yellowish color, where it isn't tasteless (metallic is a dead giveaway its ionic silver), especially if its clear as water, what they're selling is ionic silver which you don't want or worse yet, they're selling you a bill of goods and shipping you water (oh yeah some do).
Besides, I'm sure they're charging a LOT for it.
Learn to make it yourself.
Does it matter if you make 500PPM or say 120-160PPM like I do? Here at 160PPM, 1 liter of it is equivalent to 8 liters of 20PPM. Isn't that good enough? And its a lot easier to make 120-160PPM than it is to even make 320PPM. So, you make 2 liters and store it and now you have 12-16 liters of 20PPM (120PPM/160PPM). Isn't that enough? With the correct stabilizer (gelatine), it will keep basically forever.
Its not that it will go "bad" but instead, over time without a powerful enough stabilizer, the silver particles will start to stick together forming larger masses that cannot be absorbed into the body. Gelatine and perhaps other capping agents (I can't speak with authority on any other as all I use is gelatine) certainly prevent this.