Is The Mileage Club a legitimate safe business to buy miles from?
Looking at their website, their internet presence, their blog and the fact that they seem to be very active to date as they update their social media profiles very often, it is safe to assume that the business side of things is legitimate: they seem to be actively looking for customers and are doing their marketing properly. A simple google search for "the mileage club scam" reveals plenty of interesting results. Up to you to decide whether to use their services or not.
Now, from a purely legal standpoint, buying and selling miles from/to third parties is not illegal. Nevertheless, this practice is forbidden by most, if not all, airline policies. If caught you risk having your miles cancelled, and even having your account frozen or deleted. Here is what AA has to say about this:
At no time may AAdvantage mileage credit or award tickets be purchased, sold, advertised for sale or bartered (including but not limited to transferring, gifting, or promising mileage credit or award tickets in exchange for support of a certain business, product or charity and/or participation in an auction, sweepstakes, raffle or contest). Any such mileage or tickets are void if transferred for cash or other consideration. Violators (including any passenger who uses a purchased or bartered award ticket) may be liable for damages and litigation costs, including American Airlines attorneys’ fees incurred in enforcing this rule.
Use of award tickets that have been acquired by purchase or for any other consideration may result in the tickets being canceled, confiscated and/or the passenger being denied boarding. If a trip has been started, any continued travel will be at the passenger's expense on a full-fare basis. The passenger and member who attempts to use such a ticket may also be liable to American Airlines for the cost of a full fare ticket for any segments flown on a sold or bartered ticket.
Fraud, misrepresentation, abuse or violation of applicable rules (including, but not limited to, American or American Eagle conditions of carriage, tariffs and AAdvantage program rules) is subject to administrative and/or legal action by appropriate governmental authorities and American Airlines. Such action may include, without limitation, the forfeiture of all award tickets and any accrued mileage in a member's account, as well as cancellation of the account and the member's future participation in the AAdvantage program. In addition, American Airlines reserves the right to take appropriate legal action to recover damages, including its attorneys’ fees incurred in prosecuting any lawsuit.
American Airlines reserves the right to audit any and all accounts at any time and without notice to the member to ensure compliance with AAdvantage program rules and applicable conditions of carriage and/or tariffs. In the event that an audit reveals discrepancies or violations, the processing of AAdvantage awards, mileage accrual and summaries may be delayed until the discrepancies or violations are resolved in a manner satisfactory to American Airlines. Pending such resolution, members may be prohibited from redeeming mileage credits for an AAdvantage award or ticket as determined in American's sole discretion.
United says something very similar:
Prohibition of sale or barter
8. The sale or barter or attempted sale or barter of any such mileage, certificates, awards or benefits other than as authorized and/or sponsored by United is expressly prohibited. Any mileage, certificates, awards or benefits transferred, assigned or sold in violation of the Program Rules, in addition to exposing the member to the penalties otherwise associated with violations, may be confiscated or canceled. The use of award tickets that have been acquired by purchase, barter or other conduct in violation of Program Rules may result in termination of membership, cancellation of accrued mileage, certificates, awards or benefits, confiscation of the tickets, denial of boarding with respect to the ticket holder, and, at United's discretion, completion of the travel only upon payment of an applicable fare.