Unless your bill is either
(a) a brown or yellow seal WWII note and/or
(b) in uncirculated condition,
it's probably not going to be worth trying to sell it. Despite its age these bills are very common among collectors and a dealer is unlikely to pay more than about $2 for it.
If it meets either of the two criteria, though, it would be worth enough to make selling it realistic. You could consign it at auction, sell on eBay, or simply take it to a currency dealer. You'd only get a wholesale price from a dealer so you'd have to weigh the "hassle factor" of selling it privately.
if the seal is blue then yes. it's a silver certificate.
The value of the brown seal ranges between $130 and $150. The price is dependent on the market demand and other factors.
There weren't any federally-issued $20 silver certificates dated 1909 but at that time many private banks printed their own bills under federal charter. Please post a new, separate question with more information. If your bill has the name of a private bank on it, include that name. Also note the seal color - at that time they weren't standardized so a blue seal didn't indicate that a bill was a silver certificate.
There aren't any Federal Reserve indicators or seal on silver certificates. Silver certificates were issued directly by the government and not through the Federal Reserve system.
Bills printed from 1928 till 1957 (the last series year) will have the words Silver Certificate printed across the top of the bill's front. In most but not all cases, the seal and serial numbers will be printed in blue ink. Some special WWII bills may have brown or yellow seals. Older (pre-1928) bills may be more difficult to identify. Some will say Silver Certificate, but others may use words such as "5 silver dollars" or "one dollar in silver coins", etc.
The blue seal indicates your bill is a silver certificate, a form of paper money issued until the early 1960s. Please see the question "What is the value of a 1935 A US 1 dollar silver certificate?" for more information.
The blue seal indicates your bill is a silver certificate, a form of paper money issued until the early 1960s. Please see the question "What is the value of a 1935 A US 1 dollar silver certificate?" for more information.
The banner across its top and the blue seal indicate it's a silver certificate, a form of paper money that was discontinued in the 1960s. Please see the question "What is the value of a 1935 F US 1 dollar silver certificate?" for detailed information.
The banner across its top and the blue seal indicate it's a silver certificate, a form of paper money that was discontinued in the 1960s. Please see the question "What is the value of a 1935 F US 1 dollar silver certificate?" for detailed information.
Normally it would be necessary to have its denomination but the only bill fitting that description is a $1 silver certificate. Please see the question "What is the value of a 1935 B US 1 dollar silver certificate?" for detailed information.
Please check your bill again and post a new, separate question. All 1935 D silver certificates had blue seals. The last red-seal $1 silver certificates were printed in 1896.
The blue seal indicates your bill is a silver certificate, a form of paper money issued until the early 1960s. Please see the question "What is the value of a 1935 A US 1 dollar silver certificate?" for more information.
The banner across its top and the blue seal indicate it's a silver certificate, a form of paper money that was discontinued in the 1960s. Please see the question "What is the value of a 1935 C US 1 dollar silver certificate?" for detailed information.
The blue seal indicates your bill is a silver certificate, a form of paper money issued until the 1957 series. Please see the Related Question for more information.
The banner across its top and the blue seal indicate it's a silver certificate, a form of paper money that was discontinued in the 1960s. Please see the question "What is the value of a 1935 C US 1 dollar silver certificate?" for detailed information.
The banner across its top and the blue seal indicate it's a silver certificate, a form of paper money that was discontinued in the 1960s. Please see the question "What is the value of a 1935 E US 1 dollar silver certificate?" for detailed information.
The banner across its top and the blue seal indicate it's a silver certificate, a form of paper money that was discontinued in the 1960s. Please see the question "What is the value of a 1935 D US 1 dollar silver certificate?" for detailed information.