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I recommend having several bookmaker, and especially exchange accounts. This is useful to obtain the best prices, and to cover yourself in case of outages. It is also important to trade in liquid markets. This means markets where there is money available to back and lay at prices which do not have large gaps between them.
As a rule of thumb, live video matches on Betfair have good liquidity where you can get large trades matched. The matches without live video tend to be fairly mixed in terms of liquidity - you need to rely on a live score website such as flashscores.co.uk, and get your trades in quickly before the service game starts. Liquidity during individual games generally is not great. Ideally you’d want to have at least £30,000 to £40,000 matched before the match starts for it to be a viable candidate to trade in-play.
This is particularly relevant when your stakes are reasonably high and also if you want to trade points in-game. I’d only recommend live streamed matches where at least £100,000 is matched (or it’s on a high profile television channel) if you want to trade points in-game.