If it were me, I would have your agent call both of the buyers' agents and let them know there is an another offer. 

I have a buyer but he said it’s worth 40k and he can close immediately, however I have it under contract for 73k the buyer said he will give me 5k as a finders fee.

Questions:-should I stay away from investing in a town home due the common risks (smaller buyer pool, potential HOA issues, large competition etc)?

The fees for a Buyer's agent are paid by the seller, not by you.

Second, and perhaps most importantly, it's land-locked behind another property, so I don't even know if any buyers would even have access.

In our area, 3% deposit will immediately go to Escrow, preapproval from local major lender is carefully contacted with verbal confirmation that the buyer stands out. 

The websites are built and I have a small cash buyers list built ( not sure how good yet ) but, I don't want to waste money on advertising that's not going to work or buying some software that won't do me any good.

I also don't tie my investor buyers into an exclusive buyers agreement mainly due to this sort of second market. 

I sold a property and gave the buyer a deed.

I know sometimes the seller and buyer kind of splits the closing costs, but Does that 3% account for just my half or all of the closing costs (even seller costs).Because I plan on including multiple offers in my offer letter and one includes that I will pay for all closing costs.. is that already assumed that I should or usually how does it work, splitting the closing costs?