You can buy just about anything,but this page concentrates on books and computers*.Some of these sites sell more than that.

Computers

We're not carrying name-brand manufacturer sites,you should be able to find those on your own...

Dealers

Neptune Small manufacturer that guarantees lowest prices.
(Link is down...they seem to have either sold their buycomputer.com domain name to,or had it hijacked by,)
Buy.comAn emporium of sub-sites.
NECX(ConvergeTrade)
Sparco Last 2 sites not now retailinf

Fry's Cyberian/Outpost has folded into this Online superstore.

PC Mall
Computability Recently acquired by PC Mall,includes consumer electronics.
Computer Discount Warehouse
has now acquired much of Warehouse.com
(aka MicroWarehouse,DataCommWarehouse,MacWarehouse,ComputersByNet...)
PC Connection
TigerDirect
MEI Micro Center
Global Computer
(Cozone)
Catalogue dealers' online presences.
Some formerly listed small dealers have disappeared.

PC Parts Inc.
Computers4Sure has folded into Officemax.

Comparison Services

Comparing dealers' prices is a specialty on its own...

Shopper.comFrom CNET
KillerappAlso owned by CNET
PriceScan Manually updated,most of these are search engines.
Pricetrac(Letssell...a secure URL)
Dealtime
Excite Product Finder
MySimon
ComputerShopper (from ZDnet)
Reseller Ratings They compare service,not just prices.
GomezA more general version of RR.
Bottomdollar(FreeMerchant) now merged with
PriceGrabber Another generalist.

Auctions

Surplusauction Putnam Internet has done a lot of buying here.
Onsale
Sites now merged.Actually both flat-price dealers and auctioneers.
Ubid Eclectic auction dealers.
Bargainbid
CNET auctions
Fairmarket Several auctioneers in one.
Ebay Not dealers,they match buyers and sellers(for just about anything).

Auction Comparison Services

Auctionwatchers
BidXS

Books

New Book Dealers

Amazon Known as a bookseller but diversifying like crazy.
Books-a-Million Online bookstore.
Barnes and Noble Offshoot of a large bookstore chain.
Borders
Powell's A large independent bookstore's online presence.
Tattered Cover Another large independent(despite name,NOT a used-book store).
Strand Bookstore A large independent in New York.
Edward R. Hamilton Remainder dealer.
Daedalus Books Remainder dealer.
Tobypress A small publisher that does direct,exclusive marketing.
No Amazon was a site for people who don't like Amazon.It's gone.

Used Book Sources

Bookfind(aka Book Search) Site down
Bookfinder
Bibliofind(folded into Amazon)
Advanced Book Exchange now Amazon-owned
Alibris"Books You Thought You'd Never Find".
Book Avenue(site down)
International Book Collectors' Association Links to get books,but also a lot of information.

Search/Comparison Sources

AddAll Searches some,not all,of the sources above,and a couple of others,to compare prices.
Allbookstores Another comparator,with a backup site too.
BestWebBuys
Direct Textbooks Specializes in textbooks.
Fetchbook

*What were you expecting...sexy lingerie?
(Oh come on now!!!)