Fix buy_price_coef inversion

buy_price_coef isn't a direct percentage for their buyback value, it's
an inverse percentage

If it is 30, then their buyback rate is 70% of the handbook, not 30%

Libraries/SPTarkov.Server.Core/Helpers/TraderHelper.cs

@@ -578,7 +578,7 @@ public class TraderHelper(
             // Get loyalty level details player has achieved with this trader
             // Uses lowest loyalty level as this function is used before a player has logged into server
             // We have no idea what player loyalty is with traders
-            var traderBuyBackPricePercent = traderBase.LoyaltyLevels.FirstOrDefault().BuyPriceCoefficient;
+            var traderBuyBackPricePercent = 100 - traderBase.LoyaltyLevels.FirstOrDefault().BuyPriceCoefficient;
 
             var itemHandbookPrice = _handbookHelper.GetTemplatePrice(tpl);
             var priceTraderBuysItemAt = _randomUtil.GetPercentOfValue(traderBuyBackPricePercent ?? 0, itemHandbookPrice, 0);