I do agree , that a newbie , should not buy from a swap meet. However , the best running engine I have is a Saito .72 , I bought off of E-bay for $85.00 shipped.
I also pick up a Magnum 1.20 4 stroke and a World Models Clipped wing Piper Cub with floats. this weekend at a swap meet. Had to replace an o-ring on the intake manifold of the 1.20 and now it runs great.
Out of my 13 engines , 6 are used , 7 are new , and all run great. And I only buy used ring engines , never ABC , in case they do need attention, I can get by, buying a ring, but have`nt had to yet. Most just need a little TLC to get them running, generally , it`s air leak problems.
I do have one advantage over some people, as I`ve rebuilt engines of every size and application most my life and know what to look for. Like a vendor putting thick oil in the cylinder to raise the compression .
The easiest engine I have had , is a new O.S .46.AX , This is a good engine for a newbie , and is a no brainer. But you pay for it. The cheaper engines are just as good if you break them in right , but take a lot more tuning than the O.S. The cheaper engines , seem to have more power than the O.S once they are tuned right.
I just broke-in a GMS .47 , it took alot of fiddling and tuning to get it right but I was rewarded with a very strong running , high rpm engine for under $50 .
Bottom line, Buyer beware, when it comes to used.