The 'other companes' are selling on plates that are already issued - usually on behalf of owners who want to sell. Prices should be negotiable to some extent. Occasional auctions through some of the more upmarket companies, usually at classic car auctions.

DVLA only sell previously unissued plates. Mostly by static price (non-negotiable), but they also hold regular auctions for the more pricey end of the market.

It's worth noting that some of the 'other companies' are not below showing unissued DVLA plates on their sites, usually with a hefty mark-up from the DVLA price (which you could get by going to the DVLA site).

At the end of the day though, it's all about who has the specific plate YOU want for sale (aasuming you want a specific plate). I've got a 'xx ROB' plate, and bought it through DVLA auctions, mainly because there are a tiny number of plates with my name on the market. My wife (Caroline) has a 'CAZ xx' plate on her Shogun, which we bought through one of the private companies (possibly Newreg, but not sure, it was a few years ago).

In both cases, the transaction was seamless.