* Also UK focused - Matched Betting. Exploiting Bookmakers signup/recurring offers for a guaranteed profit. This is good for an early surge of ~£2k followed by a pretty easily achievable £200-£500 per month for <5 hours commitment per week.
What I interpret your explanation as is more akin to trading - Buying backs at a time and waiting for odds to fluctuate at an exchange for a sale.
Although there is no (simple) way of doing a pure arb between a bookie/exchange, all the popular exchanges expose existing liquidity in a market and you can quite safely reduce the risk by double checking available liquidity at the exchange in the seconds before placing your back bet.
There is an upfront time element needed, and then to keep it rolling you'll need to take advantage of reload offers if you want a trickle, but if you're prepared to put the work in and use a site like this that does most of the "thinking" for you, it can work.
I remember the now defunct site that showed me how to bag bonuses as a student a decade ago being straightforward about your pretty modest level of expected winnings (and for blackjack bonuses, modest level of risk) and focusing mainly on the actual maths.
For anyone that wants it as a passive income project, there's some very nice potential affiliate earnings (redirected through another domain, naturally) for anyone that wants to create a less scammy looking explanation...
If you're interested I highly recommend the MoneySavingExpert Matched Betting forum - http://forums.moneysavingexpert.com/forumdisplay.php?f=41
It is free, unbiased (lots of the specifically tailored sites like PA mentioned above have affiliate deals which reduce the amount you can earn) and active.
Discussion on the MSE forum is restricted to what they define strictly as "risk free", not "EV+" opportunities, but that's a healthy start.
Their definition of risk free is making a guaranteed profit regardless of the outcome - Either natural arbitrage or an offer-dependent forced arbitrage.