I don’t know if there are any endpoints available so I’ll propose my solution as a last resort if all else fails. You can probably scrape the item’s webpage. Inside the “item-container” section, there’s a field “data-expected-price” which tells you the expected price (best price) of an item. In the case there’s no resellers (for example Fake Chartreuse Periastron Gamma - Roblox), the price will be 0. Items can’t be resold for less than 1 robux so you know for sure that if an item’s expected price is 0 it definitely doesn’t have any resellers.
You’d just make a request to the website, it’ll return a string which is the HTML of the website. Just string match for ‘data-expected-price=“a number”’
I’m not sure. Personally I opt to make my own proxy server using ProxyService but some open sourced proxy servers exist (first that comes to mind is roproxy)