Both answers are kind of correct, but dont seem to answer the question. I think a better answer would be to explain the process. When you buy a suppressor it is transferred to a local dealer on a form 3. You complete the form 4 with the local dealer and send a check (or add your credit card # to the form 4) to the BATFE. So no, you cannot buy a stamp from silencershop for a suppressor on Gunbroker. The silencershop stamp is simply prepaying for the stamp when you purchase a suppressor from them.

I would check with the local dealer you plan to transfer to. Transfer fees can be very high. Unless you get an amazing deal on Gunbroker, it may be better to simply purchase a suppressor from a silencershop dealer to avoid additional transfer fees.