I always use PayPal for my transactions, there's no reason PayPal wouldn't work well for you if you have a legitimate PP account and are a legitimate seller. Create a site for your IG page, post your URL in your bio and integrate a "Shoutout Form" on your site that will allow users to enter their details and check out. You want to present yourself as a brand/business, be professional and you'll get advertisers (Assuming your IG page has a good follower base & a high engagement ratio).