I just removed the sewn on tag from a couple YSL pocket squares my father gave to me the other day. To be honest with something like a pocket square I don't know you should be that interested in resale value. With the exception of a few brand or material purists (I assume they exist), most people purchase a PS based on its look. Things that increase the value like nice silk and hand-rolled edges are fairly evident but even those are perks IMO. I generally buy a PS because I like the pattern or know it will match an outfit I have in mind. All that said, my YSL pocket squares had a sewn on tag and featured a logo printed into the silk. IME most higher-end PS makers (Hermes, Talbott, Versace, etc) all print their logo in the silk somewhere. So if it bother you that much perhaps look for the logo somewhere in the pattern? If the tag is interfering with your ability to fold or roll the PS then I say get rid of it. Never base your clothing or accessory decisions on their resale value. It's your item. Enjoy it in good health.