After some trial and error I finally wrapped my head around OAuth and the way Etsy implements it. The api_key parameter is only to be used when you're calling a none OAuth required method. Otherwise you have to send it all the required OAuth params. Below is working code. I leveraged RestSharp, as well as this OAuth base I found here. Hope this help some poor sap from staring at crappy code for 3 days (like yours truly).
var restClient = new RestClient(baseUrl);
OAuthBase oAuth = new OAuthBase();
string nonce = oAuth.GenerateNonce();
string timeStamp = oAuth.GenerateTimeStamp();
string normalizedUrl;
string normalizedRequestParameters;
string sig = oAuth.GenerateSignature(new Uri(baseUrl + MethodLocation), consumerKey, consumerSecret, Accesstoken, AccessTokenSecret, "GET", timeStamp, nonce, out normalizedUrl, out normalizedRequestParameters);
// sig = HttpUtility.UrlEncode(sig);
var request = new RestRequest(MethodLocation);
request.Resource = string.Format(MethodLocation);
request.Method = Method.GET;
// request.AddParameter("api_key", consumerKey);
request.AddParameter("oauth_consumer_key", consumerKey);
request.AddParameter("oauth_token", Accesstoken);
request.AddParameter("oauth_nonce", nonce);
request.AddParameter("oauth_timestamp", timeStamp);
request.AddParameter("oauth_signature_method", "HMAC-SHA1");
request.AddParameter("oauth_version", "1.0");
request.AddParameter("oauth_signature", sig);
restClient.ExecuteAsync(request, response =>
{
var r = response;
});