There's only one thing that matters : how many of the buying public know about, and can type in without needing to look up, your website address.

Autotrader is very expensive, I know (a friend in the trade). But it gets so many hits and searches that it's become almost the default site to go to.

motors.co.uk is similar - a bit less well-known, the search function on here goes through it.

then ebay motors. Some good, some dross.

Then gumtree, free-ads type sites.

I've never even heard of the site you're looking at.

Firstly, you have to think where you buy your stock from - if you haven't looked at a particular website, or not had much success with them, then the chances are that your customers aren't going to find any joy there either. Likewise, having your own website is probably a waste of time unless you're already a well established trader - you have to think how people will end up at your website, and in the case of most potential purchasers, this would be via a link from wherever your vehicle(s) is/are advertised, ie Autotrader, Ebay, etc.

Autotrader is expensive, but has a lot of viewers, which unfortunately also means there's a lot of competition in terms of cars for sale. I've never heard of motortradersales, which suggests that it would be a gamble as to how many potential customers would see your vehicles. A website analysis search shows it to have virtually no traffic, which doesn't bode well...

I find Ebay good for both buying and selling to be honest - the feedback system generally gives both buyers and sellers some insight into whom they're dealing with. Like SLO, I've never really had any success with Gumtree: it seems that there are a lot of time wasters on the site and looking on there for new stock or trying to sell there is a time consuming and usually fruitless excercise in my experience.