The following are most of the Ebay resources I've used over the last few years. They are listed by their Ebay user ID within the categories.
antlersexpress - good source of claro walnut. Most of their stuff will need to sit in your shop for a while, since they sell most of their stuff in partially-dired condition. Some of their claro walnut is just gorgeous, and they have curly redwood and redwood burl too.
bellforestproducts - Maple. Some of the best birdseye I've seen or bought. Located in Northern Michigan, their birdseye is hard rock maple, but their curly maple is from red maple trees, and the wood is not as hard or stable as hardrock maple. I have made a couple of strat-style necks from their curly red maple with excellent results. I aslo made a neck for an acoustic 12-string neck from it, and the resulting neck is PRIMO!
burl.quilt - Mostly figured maple bookmatched tops - they have quilted and flame maple drop tops for very reasonable prices - and it's good stuff.
buzzsaw-international - beautiful figured and spalted maple, but expensive
cook-woods - All kinds of exotic woods. They tend to be expensive
fine_woods - spalted and figured maple tops & billets.
lewoh - curly maple boards, mostly red maple, sutiable for drop tops mostly.
mackieorchids - koa from Hawaii. Very nice stuff, some of it (of course) very expensive.
nicemaple - figured and spalted maple tops
northcoasthardwoods - maple & walnut
renobird - mosttly redwood - curly billets and redwood burl
sandi1070 - bookmatched maple & walnut tops - spalted too
woodbay - figured maple and some koa
veddermountain - maple bookmatched tops, some spalted
woodshed44 - I've gotten some good hard maple here. Their focus seems to be furniture builders, but I've gotten some good quarter-sawn hard maple from them - great for making Fender style necks. Also some nice black walnut.
aiguitars - parts for electric guitars mostly
bezdez - lots of acoustic and electric guitar parts, supplies. Much of it is new old stock.
choppersmusic - electric guitar & bass parts. Pickups too.
flex-abc - located in Singapore. Lots of electronics - preamp modules & stuff, pickups. Tuning machines & other electric guitar hardware. They carry Belcat and Artec electronics for electrifying acoustic guitars
guitarheads - parts for electric guitars
ldill101 - guitar parts including pickups, as well as bodies and necks
proguitarshop - guitar parts, including bodies and necks
superiorbass - they specialize in bass parts & pickups, but they have stuff for guitars too
warpdrivemusic - guitars, basses, and parts. Also a supplier of rejected wood acoustic guitar parts from the Martin factory - necks, tops, bridges, fingerboards, you name it!
wildberryskittles - guitar parts & pickups
fretdoctor - guitar builder's mostly, some supplies
guitarmakingstuff - guitar builder's tools & supplies - lots of inlay stuff
tippie53 - builder's tools, mostly for acoustic guitar builders