Hi there,
I've worked in setting up a new small business before, and here's how we expanded. This may or may not be useful to you, but you might glean some ideas from it all the same. 🙂
1 - Put some of your core stock & prime pieces onto pre-existing sites. Afraid I don't know any that are particularly for coin sellers, beyond eBay. Remember you may have to increase prices to cover (in part) their commission. (We were home interiors, so we were Etsy, Amazon Handmade, Not on the High Street etc)
1a - Many of these sites will have an integration system for quick postage labels.
2 - When selling stuff, include a flyer or business card with your direct site. Make sure your prices are cheaper and/or have a better selection. Or have a discount code, or free shipping. Whatever you feels best for you. Don't be too excessively generous; you don't want your customer to feel you ripped them off on the first sale.
This encourages repeat online buyers to go direct.
3 - Set a small advertising budget on your most successful third-party outlet. Expect to lose money initially as you tweak & refine. Over time, this should increase initial third-party sales, which should then increase your direct repeat sales.
4 - As web traffic to your direct site increases, if you're really confident in your advertising ability, Google Ads. In all honesty, for such a huge market as coins, you're fighting some seriously big boys. So unless you're very niche, you'll struggle there.
=====Other Tips
A - Always try local coin fairs, town markets, things like that. If you're specialist, go to related events. E.g. Go to Renaissance fairs if selling medieval coins. Remember your business cards/flyers! Do that whenever possible & convenient.
For coin events, check Numista's own “Outings” tab. You'll likely find a few there. 🙂
B - Be quick & responsive to customer enquiries. Answer directly & casual friendly. If you have to deliver “bad” news, try and work for another solution. People love great customer service.
C - This is more a personal belief, but don't do limited time discount sales. Offer fair pricing all year rather than inflated pricing most of the year so you can do a 25% off sale in November.
If you have clearance stock, fine, sell that at marked-down pricing.
D - Having a social media presence is great. You don't need to be constantly on it with posts - keeping it few for big things. E.g. Just added over 500 Spanish coins, why not check them out?, or look at this super-rare piece I have. Maybe a couple posts a month. Try to hit about a week before most people's payday (Get them excited & they might budget for it), then maybe follow-up on payday.
But it's great for customer communication. E.g. People might ask if you can acquire certain pieces, or provide extra photos.
I do hope that might give you a few ideas. I know every business & business owner's life is unique, so some stuff might be useful. Some might be junk. 🙂
Best wishes on your new venture.