A troy ounce is slightly heavier than an avoirdupois (United States) ounce, weighing 1.097142857 avoirdupois ounces. A troy ounce is also 31.1034768g (versus an avoirdupois ounce that weighs 28.3495231g).
Large quantities of silver are usually measured in millions of ounces (for example, a silver ETF may contain 300 million ounces). Occasionally, tons will be used (although tons are more commonly used with gold). A ton of silver always means a metric ton, which weighs 32,150.7466 troy ounces.
You may occasionally see silver sold by the 'pound', but this is almost always done by individuals or inexperienced dealers (or scam artists). That's because the times that pounds are used, it usually comes about when an individual weighs the silver on a bathroom or postage scale (using avoirdupois pounds and ounces). So they are probably referring to an avoirdupois pound, which actually weighs more than a troy pound (an avoirdupois pound is 16 avoirdupois ounces, whereas a troy pound is 12 troy ounces). But, they could be referring to a troy pound. A scammer selling a 'pound' of silver will typically sell a troy pound to someone who thinks that it is 16 troy ounces of silver (when in fact it is 12 ounces). The scammer avoids any legal problems, because a pound of silver is indeed 12 troy ounces.
|1 Troy Ounce =||1.097142857 Avoirdupois Ounces|
|1 Troy Ounce =||31.1034768 grams|
|1 Troy Pound =||12 Troy Ounces|
|1 Kilogram =||1,000 Grams|
|1 Kilogram =||32.1507466 troy ounces|
|1 Avoirdupois Ounce =||0.911458333452013 Troy Ounces|
|1 Avoirdupois Pound =||16 Avoirdupois Ounces|
|1 troy ton =||2450 troy lb.|
|1 Metric Ton =||1,000 kilograms|
|1 Metric Ton =||32,150.7466 troy ounces|