GSA eBuy – You need a login to access the opportunities on this site.
GSA Advantage – Where your product are listed for Agency Buyers to order. This is also a Search Engine for Services.
FedBizOpps – FBO is where Open Market opportunities are posted, this is worth tracking. GSA-only opportunities can be found here as well.
FAS Schedule Sales Query – A tool to research GSA Sales information, great for Competitive Research.
GSA eOffer – Where New offers and Modifications are uploaded.
FAS Sales Reporting System – this is where you report sales (whether you are participating in TDR or not).
Vendor Support Center – the General site to bring you the information you are looking for.