Sears, Roebuck and Co. Hercules Work Clothing - NOS Checkered Shirt

[The Vintage Vendeuse]

I frankly admit that I don't think I've bothered to read every eBay seller's TOS when I make a purchase... in fact, I'm not sure I've ever read a seller's TOS... and I think that's fairly common. If I like and want an item, I simply just try to buy it. I figure the system will stop me if the seller has rules preventing my purchase.
It sounds like you have some potential buyers who are very enthusiastic about your item, they may not be "intentionally" trying to violate your rules.
I've also felt rather complimented when I receive inquiries about ending an auction early, though I rarely did so... it's exciting to know I have a popular and desirable item. I also feel it's natural for a buyer to try to get a good deal and perhaps make rather lowball offers just to see what happens.
Personally, I consider this all somewhat common buyer behavior and I don't assume the buyers are "suspicious".

(I still want to cry over some Royal Doulton 1950s china I missed out on. About 15 years ago, I purchased a lovely 1950s tea set for four which was a simple, but elegant pattern I had never seen before, and was not shown on the replacements website. Seller willingly shipped it from England to me in the USA. About five years later, I found a different seller in England had listed the matching berry bowls! I excitedly hit the BIN button and it blocked me, the seller didn't ship to the USA. I checked their other dozen or so listings, all of different china and all eligible to ship to USA. I politely messaged the seller, thinking the listing for my berry bowls had mistakenly blocked US buyers. No, the seller responded by changing all their listings to block US buyers. Over a decade later, I still feel the frustration of not having those darn berry bowls! I was willing to pay the asking price and any appropriate amount for shipping but the seller stood firm. )

 

[A.O.T.]

I figure the system will stop me if the seller has rules preventing my purchase.
It sounds like you have some potential buyers who are very enthusiastic about your item, they may not be "intentionally" trying to violate your rules.
I've also felt rather complimented when I receive inquiries about ending an auction early, though I rarely did so... it's exciting to know I have a popular and desirable item. I also feel it's natural for a buyer to try to get a good deal and perhaps make rather lowball offers just to see what happens.
Personally, I consider this all somewhat common buyer behavior and I don't assume the buyers are "suspicious".

Unfortunately eBay is only concerned with making their commission off the sale and not the welfare of the seller.

Within your eBay shipping preferences you can exclude what areas you do not ship to.

https://www.ebay.com/ship/prf

You can override these settings within individual listings. In my case this would mean overriding a setting in a listing within GarageSale, which is a Apple desktop inventory application for eBay. I've used this and similar listing software for eBay since 2001 since its the fastest way to list items.

Many sellers are blocking shipments to various countries because they don't want to accept the additional risk that comes with international shipments and buyers outside of U.S. law. Should a scam occur there is no legal recourse to be taken against the buyer.

Other sellers are motivated by not wanting to deal with the additional requirements of shipping - be it direct, through eBay's reshipping program or through a 3rd party reshipper.

Most sellers, especially newbs, will falsely assume that blocking these countries means buyers from these countries can't buy your items.

But of course that is not true due to eBay's greed.

eBay lets buyers in countries you've blocked buy your item anyway simply by changing their shipping address to an address in a country that you do ship to. For U.S. sellers this typically means a buyer is using a reshipping company located in U.S., typically near a airport.

The buyer's country is listed on the listing's list of blocked countries and the buyer intentionally uses a false shipping address in the seller's country to bypass the seller's wishes not to sell to anyone outside the U.S.

Thus any seller on eBay needs to add a TOS to their listings to counter the attempt by eBay and a buyer to bypass the seller's terms.

Thus, if you have all other countries blocked you should add to your TOS:

Buyers must be U.S. citizens.

Buyers must reside within the 50 U.S. States, a U.S. Protectorate or have a APO/FPO/DPO military address.

No shipments to a forwarding service; a "gift" addresses; outside the 50 U.S. States, a U.S. Protectorate or a APO/FPO/DPO military address.

Now when these buyers from outside the U.S. intentionally bypass your eBay settings which block their country and bypass your TOS you can cancel the transaction and eBay will remove any retaliatory feedback the buyer leaves for not selling the item to you.

If you don't include this in your TOS you may be looking at a mark against seller performance rating as well as a negative feedback.

Similar shirts which are used, or which don't have their condition noted, have prices of over $500 so offers for $75 to $300 are from resellers and not end user collectors. Surely a NOS shirt would be worth more.