I've been researching prices and platform over the last few months, as I am also on the threshold of buying into Vintage MTGO.
I also strongly endorse mtgotraders. I have used the coupon code "numotpaypal" (without quotes) for the same discount, which I believe is 8%. Note that the discount is available only if you are paying cash via paypal (versus with mtgo event tickets). Right now, the mtgotraders "sell" price for black lotus, with the discount, is $133.11. That is actually slightly less then their advertised "buy" price in tickets(!), and is lower than the price price listed on cardhoarder, cardnexus, dojotradebots, or GoatBots (cardhoarders was the second lowest, with a cash price of $136).
If you haven't bought cards from one of these sites, the way it works is you complete the transaction on their website via paypal, with your mtgo account open. You give them you mtgo user name, and their bot initiates the trade within the game. It is surprisingly fast and easy. If you want to do a test run, you can buy a low value card to see how it works.
Note that their mtgotraders does not have the best prices if you are paying in tickets, whether you initiate their transaction with their tradebots or on mtgo. For that, I've found cardnexus, goatbots, or dojotradebots seems to be best. But I haven't yet won enough tickets for that to be an issue for me Also, cardhoarders and mtgotraders have larger selections, and are more likely to have what you need.
You can find deals from individual users if you are patient, but the MTGO trading platform and makes it hard for individual users to find each other among the sea of bots. And you'll never pay less for a card than the advertised buy price of the major websites, so there isn't a whole lot of value to be saved, especially for premium cards that have a relatively thin buy/sell margins.
Good luck, and hope to see you on MTGO soon!