My studies don't really allow me to have a regular job, so to make a bit of extra cash, I buy stuff from car boot sales/ebay and sell them on (usually on ebay). Here's some examples from the last couple of months - the kind of thing I look out for:

1. 1970s flip clocks. I bought a Copal at a car boot sale for £3, and I sold it on ebay for £40. I've got another two that I bought for less than a fiver each which I'll sell in the Spring

2. 1980s Braun stuff. Their alarm clocks turn up quite regularly (I've traded three and kept one for myself). I usually get them for £1 or less, and sell them for £20-30 on ebay or retro/design forums

3. Old car radios. I've bought one for £5 and sold it for £60, and I found another one in a bin (?!?!) and sold it for £30. I hadn't tested either of them, so if you knew that it worked or you were able to fix it, then you'd get more money for it.

4. Swatch watches. These are great because they were very cheap back in the 80s and early 90s, and most people just don't know what they're worth. I regularly buy Swatches for 50p, fit a new battery, clean them up and sell them for £10-15. Twice I've found a famous model (a Pinstripe) and sold it for £40.

5. 80s Audio. I don't know anything about audio, really, but generally if it looks cool and it costs less than a fiver, I go for it. I've sold three things for £20-30 and gave one to a friend for a birthday present (he likes retro stuff).

6. Art glass - you need to do a bit of research before buying and selling art glass. A lot of stuff you see at boot sales is worthless crap. Occasionally you come across something priced as worthless crap but actually worth a lot of money. I found a Whitefriars 'Bark' vase as pictured above, for £1, and sold it for £60. I can currently only identify two or three famous pieces of art glass, so I'm buying some books on the subject so hopefully I'll make some more money from it in the future.

7. Dualit toasters. I've found three of these at car boot sales, all 'broken', and bought for £1 or less. Two needed replacement fuses, sold for £12-15 on ebay. Another needed more repairs, I couldn't be bothered so I stuck it on ebay and still got £6 for it.

8. Technical drawing stuff. It's much more expensive than you'd think! Pens, drawing boards, curves/guides etc can all be found at car boot sales. I have a set of Rotring pens, completely unused in their original box in mint condition, bought for £5, and they're worth around £40-50. I've also got a large drawing board for a tenner and sold it for £40.

9. Vintage bicycle parts - again, like art glass, you need some knowledge to filter out the gold from the crap. I love cycling, bikes and vintage bikes in general so I've got a lot of experience. Things I've found include the above Campagnolo hubs, bought for £1, cleaned and polished and sold for £90 (my biggest profit), and generally parts bought for a quid or less and sold for £15-20. If you don't want to do all the research first, basically, if it's got Campagnolo/Sugino/Cinelli/3ttt written on it and it's a couple of quid, I'd buy it. If you find a leather saddle marked as a 'Brooks', they're usually worth a minimum of £20 even in relatively poor condition. Nice ones, like mine, are worth around £80.
EDIT:
10. Vintage Cameras

This is something I've just into recently. I bought the above Zenit-E on ebay for £15, then a case for £5. Went to a car boot sale and I must've seen about five or six of the fuckers, all with cases, all for less than £5. I bought three (all the working ones). One I gave to my girlfriend, the other two I sold for about £15. I don't know a lot about cameras really, but generally it seems that rangefinder cameras aren't worth a lot on ebay, but cheap SLRs (like the Zenit) are worth at least a tenner, or a fiver if broken, and the lenses are worth at least £2-3. I've also found a Kodak Baby Brownie box camera from the 1930s for £1, seen them for sale on ebay for £20, so I'll be selling that soon.
Other recent finds were a load of Star Trek annuals from the late 60s to late 70s (must be about 12 of them) for £2 the lot, they should sell for about £5 each. I also found a James Bond Lotus Corgi toy car in box for 50p, worth about £15-20 on ebay. And half a dozen 8mm film reels (mostly 1950s/60s cartoons) in original packaging for £1 (not sure what they'll sell for, some of them are quite scabby).
So that's generally the kind of stuff I look out for. I normally sell on ebay, or on specialist forums (like design forums, retro bike/car forums (as members are usually interested in other retro stuff)). Since mid October I've made around £400 profit, which doesn't sound like that much, but £50 a week is as much as I'd get from a normal weekend job, but the hours are completely flexible; if I don't feel like 'working' then I don't have to. Plus I really enjoy it. Most car boot sales shut down during the winter, but I still buy some stuff on ebay.
Three areas I'd like to go into are vintage cameras, vintage records and vintage clothing, as I usually see a lot of those things at car boot sales.