There are plenty of variables that you have to consider once you’ve decided to purchase a suppressor. Among the highest should be reputation of the manufacturer. For all practical purposes, you will own it for life, and the warranty/service commitment of the manufacturer is important.
Then you should choose based on your shooting needs. Cartridge of gun(s) it will be used with is important, as is the intended use – target shooting, personal defense, coyote hunting – know your laws thoroughly – offenses may be dealt with at a Federal level. Size/weight is important for some uses. Attachment system is important – especially if being used on multiple platforms. Obviously noise reduction is important.
Typically, most manufacturers will offer several different sizes, with noise reduction being closely related to the size. So almost all of them will make something that fits your needs. That being said, I can without reservation, recommend Dead-Air products. To me, the Dead-Air Sandman S is the absolute perfect suppressor for everything other than handgun use. Their attachment system is spectacular, and the mount actually serves and a very capable muzzle device when used with or without the suppressor mounted. A wide variety of endcaps, flash suppressors, and accessories are available for the Sandman series, and they have a stellar reputation.
Joe’s Sporting Goods has the aforementioned kiosk for Silencer Shop, but I’m not the biggest fan of either. I can recommend Hansohn Brothers as a seller, they have a website.
While it is certainly possible to sell a used suppressor, very few users are willing to deal with the headaches and legal hurdles to complete such a purchase, maybe a trusted friend would do so, but they won’t want to give you top dollar, so go into this purchase with every intention to keep it forever. And while it’s unlikely you will ever have a legal issue, remember you are dealing with the Department of Alcohol Tobacco and Firearms, and they can destroy your life. It’s extremely unfortunate that suppressors are not celebrated, and instead, are treated as evil, scary devices. Hopefully that changes soon.